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Page 66 - Substitute the figure given below for 
that appearing on the left-hand side of Fig. 6-44, 

Page 67 - Right-hand column, 4th paragraph, 9th line 
which now reads "pressed into the fixture body C, and 
is threaded with" should instead read "pressed into the 
nose piece B t and is threaded with." 

Page 122 - Left-hand column, Fig. 10-26, transfer the 
clamp shown as "a" to "b" and similarly transfer the 
clamp shown as "b" to "a". 


Erik K. Henri ksen, M. Sc. 

Fellow of the ASME 

Corresponding Member of the Danish 

Academy of Technical Sciences 

INDUSTRIAL PRESS INC., 200 Madison Avenue, New York 10016 

Library of Congress Cataloging in Publication Data 

Henriksen, Erik Karl, 1 902- 
Jig and fixture design manual. 

Includes bibliographical references, 
1. Jigs and fixtures -Design and construction- 
Handbooks, manuals, etc, 

I. Title. 

TJ1187.H46 621.9'92 73-8810 

ISBN 0-831 1-1098-8 

ACCESSION No. „„--„)( 

7 (55£ 


, ! 2 1 oct m 



. . »»*V 


Copyright ©1973 by Industrial Press Inc ., New York, N.Y. Printed in the United States 

of America. All rights reserved. This book or parts thereof may not be reproduced in any 

form without permission of the publishers. 


The Use of Metric Units 

1 Introduction 

2 Preliminary Analysis and Fixture Planning 

3 The Fixture Design Procedure 

4 Locating Principles 

5 Preparation for Locating 

6 Design of Locating Components 

7 Loading and Unloading 

8 Chip Problems 

9 Centralizers 

1 Clamping Elements 

1 1 Equalizers 

1 2 Supporting Elements 

13 Cutter Guides 

14 Drill Bushings 

1 5 Design of Fixture Bodies 

16 Drawings, Dimensions, and Tolerances 

17 Standard and Commercial Fixture Components 

18 Design Studies I - Drill Jigs 

1 9 Design Studies II - Milling Fixtures 

20 Design Studies III - Miscellaneous Lathe Fixtures 

21 Universal and Automatic Fixtures 

22 Economics 

Appendix 1 Measuring Angles in Radians 

Appendix II Transfer of Tolerance from the Conventional Dimensioning 

System to the Coordinate System 
Appendix III The Dimensioning of Fixtures by Stress Analysis 
Appendix IV Metric Conversion Tables for Linear Measure 

























The Use of Metric Units 

Dimensions and other data are, as a general rule, given in English units and in metric 
units. In the text the metric data are put in parentheses following the English data; in 
tables the metric units are usually placed in separate columns. The accuracy with which 
the conversions are performed varies with the nature and purpose of the data quoted. 
Where accurate conversion of dimensions is made, it is based on 1 inch = 25,4 mm EXACT. 
Several tables for the conversion of inches and millimeters, feet and meters, and pounds 
and newtons are presented in Appendix IV. Precise inch dimensions, written with three or 
four decimal places, are converted, as a rule, to the nearest 1/100 or 1/J000 mm. The 
purpose is to present the result of the conversion in a manner representative of the 
equivalent level of workshop accuracy. In other cases, Lc, when dimensions include 
fractions of an inch, approximations are used. For example, 1/2 inch usually is 
converted to 13 mm. There are also cases where a fairly close approximation would be 
meaningless, and where it is more realistic to present the result of the conversion in a 
round number of millimeters. When, for example, a fixture is made with an overall 
length of 16 inches, then this dimension is obviously chosen by the designer as a 
convenient round value, and not as the result of an accurate calculation. If the same 
fixture had been designed in a metric country, the designer would not choose the length as 
16 X 25.4 = 406.4 mm but would make it an even 400 mm. Likewise, an American 
component manufacturer may market an eyebolt 6 inches in length, while a European 
manufacturer may have an equivalent eyebolt that is 150 mm, not 152.4 mm, long. 
Where an American screw thread is converted, it is to the nearest metric screw thread. 
No attempt is made to convert American standard fits and tolerances. Parts with metric 
dimensions should be designed with the ISO Limits and Fits; a collection of data for this 
system is found in Machinery's Handbook, 19th ed„ pages 1529 through 1538. 
In some cases, such as in dimensioned drawings and their accompanying calculations, 
no conversion is attempted. To write two different sets of dimensions into the drawings 
and detailed calculations would be confusing. The purpose of such calculations is to 
explain the method, rather than to illustrate one particular size of an object. Also, for 
some of the commercial components concerning a specific American product, only 
English dimensions are quoted. 

Many of the book's equations are of such a nature that conversion is unnecessary since 
they are equally valid in English and in metric units. Other equations, of an empirical 
nature, include numerical coefficients the values of which depend on the type of units 
used. In all such cases, separate equations are given for use with English and with metric 
units. In most of the numerical examples, the given data as well as the calculated end 
results are stated in English as well as in metric units. 

It should be noted that conversions have been made to units in the International 
System (SI) which is rapidly becoming the recognized standard throughout the world, 
Thus the reader will find that the newton (N) and the kilonewton (kN) are the metric 
units used for force while the gram (g) and the kilogram (kg) are used for weight (mass). 


The book is written as a textbook and reference source, and is meant to be used by the 
experienced practitioner as well as the beginner, whether he is a technician in industry or a 
college student. 

The author concentrates on three major objectives: (1) to describe the fixture 
components in full; (2) to present the fundamental principles for efficiently combining 
the components into successful fixtures; and (3) to apply basic engineering principles to 
the mechanical and economic analysis of the complete design. These three tasks are 
supported by a comprehensive description of com mercjally available fixture components, 
a four-point, step-by-step method and comprehensive check list for the design procedure, 
applicable equally to all types of fixtures, and also calculation methods for the stress and 
deformation analysis of the fixture body and its major components. The use of a variety 
of calculation methods is demonstrated by numerical examples. 

The author has avoided presenting a confusion of detailed drawings of complicated 
fixtures. Instead, there are 1 5 actual cases included, ranging from the simplest drill plate 
to some complex and quite advanced fixtures for milling and other operations. For each 
category of machining operations, there is a definition of its characteristic fixture 
requirements and one or more typical examples. In addition, the book includes the 
design principles for fixtures of the most important non-machining operations, such as 
welding and assembly. 

A number of the line drawings in the book are executed in a recently introduced 
drawing style in which two line thicknesses are used for edges and contours. The 
heavier lines indicate the contours of surfaces that are surrounded by air. 

With the dominant position of the metric system outside of the United States and the 
approaching introduction of this system within this country, metric units are used 
together with the English units throughout the book. 

Four informative appendices with illustrations should prove to be helpful to the 
reader, they are "Measuring Angles in Radians," "Transfer of Tolerances from the 
Conventional Dimensioning System to the Coordinate System," "Dimensioning of 
Fixtures," and lastly, "Metric Conversion Tables of Linear Measure." 





Definition, Purpose, and Advantages 

A fixture is a special tool used for locating and 
firmly holding a workpiece in the proper position 
during a manufacturing operation. As a general rule 
it is provided with devices for supporting and clamp- 
ing the workpiece. In addition, it may also contain 
devices for guiding the tool prior to or during its 
actual operation. Thus, a jig is a type of fixture with 
means for positively guiding and supporting tools for 
drilling, boring, and related operations. Hence, the 
drill jig, which is usually fitted with hardened bush- 
ings to locate, guide, and support rotating cutting 

The origin of jigs and fixtures can be traced back 
to the Swiss watch and clock industry from which, 
after proving their usefulness, they spread through- 
out the entire metal working industry. Contrary to 
widespread belief, the recent introduction of the 
N/C machine tools has not eliminated the need for 
fixtures; to obtain the full benefit from these ma- 
chines they should be equipped with fixtures that 
are simpler in their build-up and, at the same time, 
more sophisticated in their clamping devices. An 
example of a fixture on an N/C lathe is shown in 
Fig. 1-1. 

1. The main purpose of a fixture is to locate the 
work quickly and accurately, support it properly, 
and hold it securely, thereby ensuring that all parts 
produced in the same fixture will come out alike 
within specified limits. In this way accuracy and 
interchangeability of the parts are provided. 

2. It also reduces working time in the various 
phases of the operation, in the setup and clamping 
of the work, in the adjustment of the cutting tool to 
the required dimensions, and during the cutting op- 
eration itself by allowing heavier feeds due to more 
efficient work support. 

3. It serves to simplify otherwise complicated op- 

Courtesy of Monarch Machine Tool Co. 

Fig. 1-1. Close-up of an aircraft fuel pump body housing 
mounted in its fixture on an N/C lathe. 

erations so that cheaper, relatively unskilled labor 
may be employed to perform operations previously 
reserved for skilled mechanics. Jigs and fixtures ex- 
pand the capacity of standard machine tools to per- 
form special operations, and in many cases, they 
make it possible to use plain or simplified, and there- 
fore less expensive, machinery instead of costly 
standard machines. In other words, they turn plain 
and simple machine tools into high production equip- 
ment and convert standard machines into the equiva- 
lent of specialized equipment. 

4. By maintaining or even improving the inter- 
changeability of the parts, a jig or fixture contrib- 
utes to a considerable reduction in the cost of as- 
sembly, maintenance, and the subsequent supply of 
spare parts. 

In effect, jigs and fixtures reduce costs and im- 
prove the potential of standard machines and the 
quality of the parts produced. 



Ch. 1 

Jigs and fixtures represent an embodiment of the 
principle of the transformation of skill. The skills of 
the experienced craftsmen, designers, and engineers 
are permanently built into the fixture and are there- 
by made continuously available to the unskilled 
operator. One important goal is to design a fixture 
in such a way as to make it foolproof, and thereby 
contribute to added safety for the operator as well 
as for the work. 

Application and Classification of Jigs and Fixtures 

The obvious place for jigs and fixtures is in mass 
production, where large quantity output offers am- 
ple opportunity for recovery of the necessary invest- 
ment. However, the advantages in the use of jigs 
and fixtures are so great, and so varied, that these 
devices have also naturally found their way into the 
production of parts in limited quantities as well as 
into manufacturing processes outside of the machine 
shop, and even outside of the metalworking industry. 
The many problems of geometry and dimensions 
encountered within the aircraft and missile industry 
have greatly accelerated the expanded use of jigs 
and fixtures. 

Within the machine shop, jigs and fixtures are used 
for the following operations: Boring, Broaching, 
Drilling, Grinding, Honing, Lapping, Milling, Planing, 
Profiling, Reaming, Sawing, Shaping, Slotting, Spot- 
facing, Tapping, and Turning, A systematic master 
classification of machining fixtures according to the 
characteristics of the operation is shown in Table 1-1. 

Outside of the machine shop, fixtures may be 
applied to advantage for: Assembling, Bending, 
Brazing, Heat treating, Inspecting, Riveting, Solder- 
ing, Testing, and Welding. Such fixtures can be 
characterized as manual work fixtures and may be 
classified as shown in Table 1-2. 

This book deals essentially with the design of jigs 
and fixtures for use with metal-cutting machine 
tools. Applications outside of this area will be 
shown by a few characteristic examples. 

Design and Economy 

Jigs and fixtures are special tools in the sense that 
each tool is, generally, designed and built specifically 
for making one part only and for only one operation 
on that part. There are noteworthy exceptions to 
this general rule. Quite often, drill jigs are built to 
allow a sequence of operations to be performed at 
one location, such as drilling followed by tapping or 
reaming, or drilling to increasingly larger diameters, 
or drilling followed by countersinking or boring, etc. 
Less frequently, a fixture may be designed with 

Table 1-1. Classification of Machining Fixtures 


Rotary Motion 

Straight- Line Motion 

Single- Point Cutter 

Lathe Fixtures 

Planing, Shaping, and 
Slotting Fixtures 

Multiple-Point Cutter 

Milting Fixtures for 
Circular Feed 

Fixtures for Circular 

Milling Fixtures for Straight- 
Line Feed 
Broaching Fixtures 
Surface-Grinding Fixtures 
Sawing Fixtures 


Single-Point Cutter 

Multiple- Point Cutter 

Boring Jigs 

Drill Jigs 
Tapping Jigs 
Reaming Jigs 
Honing and Lapping Jigs 

interchangeable or adjustable inserts, such that it can 
be used for several parts of slightly modified shape 
or dimension. This concept leads logically to the 
"universal fixture," although "universal" may be an 
exaggeration. A universal fixture is constructed 
from building blocks assembled on a common base 
plate to form a fixture for one particular operation. 
After its use is completed, it is disassembled and 
then reassembled to a new and different configura- 
tion. Universal fixtures and jigs of this and other 

Table 1-2. Classification of Manual Work Fixtures 



Preparatory Operations 

Layout Fixtures 

Metallurgical Operations 

He at- Treating Fixtures 

Annealing Fixtures 

Joining Operations 

Welding Fixtures 

Soldering and Brazing 


Riveting Fixtures 

Wire-Stitching Fixtures 

Crimping Fixtures 

Assembly Fixtures 

Quality Control 

Inspection Fixtures 

Measuring Fixtures 

Pressure-Testing Fixtures 

Ch. 1 


types are commercially available. They may be less 
rigid than fixtures of one-piece design but they may, 
on the other hand, have economical advantages for 
short-run production since their components can 
be reused. 

This is generally not the case, however, with or- 
dinary jigs and fixtures of special design. Disassem- 
bly and reuse of components is ordinarily not eco- 
nomically feasible; thus, when the production is 
completed, the tooling costs must have been written 
off, hopefully, with a saving and a profit. 

Due to the specialized nature of these tools, their 
designs are as varied as the parts which they are to 
serve. There are undoubtedly not two identical fix- 
tures in the whole world. The design of these tools 
is, therefore, a task that challenges the designer's 
creative imagination and draws heavily upon his ex- 
perience and ingenuity. Nevertheless, fixture design 
is not a task reserved only for geniuses. It is gov- 
erned by rules, and these rules can be learned, mas- 
tered, and practiced by average persons. 

As evidenced by the structure of this book, that 
vast variety of possible configurations of fixtures 
can be subdivided into groups of similar shapes; the 
groups can be defined and classified, the classes can 
be systematized, and each subdivision within the 
system can be evaluated for its good and bad prop- 
erties, and accordingly assigned to its optimum ap- 
plication area. 

The design process is systematized to an even 
higher degree. It is governed by a logical, step-by- 
step procedure that is time tested and leads to a use- 
ful end result. It is a cookbook recipe. As such, it 
supports the beginner, it guides the experienced 
practitioner, and it may even be of assistance to 
the expert. 

Any mechanical design is incomplete without a 
documentation of its structural integrity; that is, a 
survey of the loads acting on the structure, and an 
analysis to the effect that stresses and deformations 
from these loads remain within prescribed limits, as 
defined by recognized factors of safety and tol- 
erances of accuracy. The penalty for underdimen- 
sioning is breakage or warpage of the fixture and is 
clearly observable. Even one, or a few such cases, 
would be a lesson to the design department and re- 
sult in an upgrading of thickness standards. The 
penalty for over dimensioning of a fixture is "only" 
excessive weight, which is more likely to go un- 

Every design activity must never lose sight of the 
fact that the purpose of a fixture is economy. Each 
design assignment will have a variety of solutions 
with different degrees of operational economy, a 

different useful life; and different costs. The de- 
ciding factor, which must always be taken into con- 
sideration, is the number of parts to be produced. 

Typical Examples 

Before entering upon the detailed discussion of 
fixture design, a sampling of different fixtures will 
be shown and described. They have been selected 
to represent two characteristic types of fixtures, 
namely, miffing fixtures and drill jigs. In addition, 
they show a considerable number of typical details 
and thus serve as introductory examples for the sub- 
jects following. 

The two principal types are shown schematically 
in Fig. 1-2. Each of the two sketches shows a part, 
typical for the operation, supported on buttons and 
clamped by a clamping device likewise appropriate 
for the purpose. The principal difference between 
the two types lies in the means for obtaining dimen- 
sional control. In the milling fixture (Fig. I-2a), the 
relative position between cutter and work is initially 
found by means of the tool setting block /, shown 
to the left, and from there on the accuracy of the 

Fig. 1-2. The principal types of fixtures. (Top) A milling 
fixture; (Bottom) A drill jig. 


Ch. 1 

work depends on the accuracy and rigidity of the 
machine tool. In the drill jig (Fig. l-2b), the tool (a 
twist drill) is positively positioned by the drill bush- 
ing prior to the start of the cut, and the guidance is 
maintained throughout the cutting process. Thus, 
the relation of cutter to work is self-contained with- 
in the jig. The reason for the need of such guidance 
is the well-known fact that a drill is a relatively 
highly flexible tool; a milling cutter is not. 

The fixture shown in Fig. 1-3 is a milling fixture. 
The part to be milled is a flat bracket / of angular 
shape, with a rectangular fastening flange 2. The 
surface to be machined is the end surface of the 
short leg of the angle. The total length of the fix- 
ture is approximately 18 inches (460 mm); the 
weight, approximately 90 pounds (40 kg). It is a 
very normal size fixture and can be accommo- 
dated on any milling machine except the very small 
ones. It would, however, take two men to safely 
lift it up on the table, but a plant that is progressive 
enough to utilize well-designed fixtures such as this 
one, would probably not depend on occasional man- 
power for a lifting job, but would provide hoisting 
equipment. Once the fixture is positioned on the 
table it does not have to be moved again and is 
bolted down. The size and weight of the part to be 
cut presents no problem. 

The fixture body J is a rather solidly designed 
casting; although it could have been hollowed out 
at a few places, the weight saving is immaterial in 
this case and would only be offset by increased pat- 
tern and molding costs. 

The face of the flange was already machined in a 
previous operation and permits, therefore, locating 
on four buttons 4 without generating a redundancy. 
The extreme left end of the bracket is supported at 
one point only by means of a sliding rest 5 operated 
by the plunger 6 and knob 7. This rest is brought 
into contact after the actual locating is completed. 
Horizontally, the flange is located on two pin loca- 

gag£ fin, pf ^-miiiinc curaa 
*—,'. V I LjH*J 

^^tM y~gnk {jag] 


{P |g|gg[^gg 

Courtesy ofSiewek Tool Co. 
Fig. 1-3. A typical milling fixture designed with extensive 

application of commercially available components. 

tors 8 projecting into previously countersunk holes 
in the bottom of the flange. Again, an additional, 
adjustable support 9 is provided against a small pro- 
jection on the back of the angle to take the thrust 
from the cutter 10; a standard end mil). Two 
straight strap clamps 11 and 12, are provided; both 
are hand operated— the one to the left is screw acti- 
vated, the one to the right is cam activated. 

The fixture is aligned to the T-slots in the table by 
keys 13 and also has lugs 14 for the holddown 
bolts. The lugs are C-shaped for easy insertion of 
the bolts so that the fixture does not have to be 
lifted once it has been placed on the table. 

The main characteristic feature of this fixture is 
the systematical use of quick-action devices for the 
application of the supports and the clamping. It 
should be noted that no spanner or wrench is needed 
and that a large number of the parts are commer- 
cially available. 


Preliminary Analysis and Fixture Planning 

The complete planning, design, and documenta- 
tion process for a fixture consists, in the widest 
sense, of three phases-design preplanning, fixture 
design, and design approval. They are listed here in 
their natural sequence, although there may be some 
overlapping in actual execution. 

The Initial Design Concept 

The design concept is, even if not yet put on paper, 
presumably in the designer's mind at an early stage 
of the first phase. As the process goes forward, the 
initial concepts are recorded in the form of sketches 
and are gradually developed, modified, and changed; 
some design concepts will be discarded and replaced 
by better ones. As a general rule, there will be de- 
veloped at one time or another, a manufacturing 
operations plan, listing among other things, the se- 
quence of operations, calling for fixtures at the ap- 
propriate places within the plan and providing the 
machining parameters, cutting speed, depth of cut, 
feed, etc., for each operation. 

It is not the purpose of this book to deal with this 
planning process; however, there may be cases where 
an operations plan is not available to the tool de- 
signer and in such instances his first step must be to 
compose the operations sequence. It is an absolute 
necessity to have the sequence finalized prior to fix- 
ture design. Whether the surfaces are rough (cast or 
forged) or previously machined makes a radical dif- 
ference in locating and clamping the part. In the 
design of a drill jig it makes a difference whether 
the holes are to be drilled before or after machining 
of the surfaces, and it makes a big difference whether 
a cylinder is machined internally first, and external- 
ly later, or vice versa, 

A fundamental rule is that the cutting tool must 
have ready access to the surface or surfaces to be ma- 
chined. The requirement is obvious, but is some- 
times forgotten at the start, and a great deal of rede- 
signing may be required when the error is discovered. 

There exists a set of general rules for selecting the 
sequence of operations. They are simple and logical, 
and almost universal; exceptions to these rules may 
exist but they are rare, and usually occur only under 
special conditions. 

These rules are: 

1. Rough machining is done before finish machin- 
ing, followed by grinding, if required. 

2. To allow for natural stress relief, all roughing 
operations should be done before any finish machin- 
ing is started; for the same reason, the most severe 
roughing operation should be done as early as pos- 
sible. This last rule has, however, one important 
modification concerning the clamping or spanning 
of the part. Since, for economical reasons, the 
"most severe roughing" operation should be per- 
formed with maximum possible feed and depth of 
cut, (and, therefore, large cutting forces), it requires 
a strong clamping in or on the machine tool. If the 
rough part offers good clamping surfaces for the 
"most severe" operation, the rule stands. 

3. There may, however, be cases where the part in 
the completely unmachmed condition has no suit- 
able clamping surfaces for a heavy cut. In such in- 
stances, it may be preferable to machine some other 
surface first, which then can serve as the clamping 
surface for the "most severe" operation. 

4. Another equally important consideration is the 
avoidance of broken edges in castings and burrs on 
ductile parts. This is accomplished by choosing the 
direction of the feed so that the cutter enters the 
material from an already machined surface. This 
rule is quite general and can be applied to parts 
with combinations of machined outside surfaces and 
holes or slots. If holes were drilled and slots were 
milled first, and the outer surfaces machined after- 
wards, then there would be broken edges or burrs on 
one side of each hole and slot. Conversely, if the 
surfaces are finished first, then the drills and slot 
milling cutters would enter the material from the 



Ch. 2 

machined surface, in accordance with the rule stated, 
and broken edges and burrs would be prevented. 
5. The rule can be stated in its generality as fol- 
lows: Surface machining comes before depth ma- 

tn the preplanning phase, the designer accumulates 
and utilizes all available information as far as it con- 
cerns the design assignment. Four areas of informa- 
tion must be taken into account; the part material 

and geometry, the operation required, the equipment 
for this operation, and the operator. At this and 
other design phases, the designer may consult ele- 
mentary lists of items to be considered. Examples 
of such lists are given in Tables 2-1 through 2-4, 
while Table 2-5 gives a similar list of the individual 
items concerning the fixture itself. Such lists may 
appear trivial, however experience shows that they 
are useful assists to the designer's memory and help 
to avoid his overlooking any significant point. 

Table 2-1. Part Description Details for Preplanning of Fixture Design 

/. Material Class 

1.1 Casting 

i.2 Sintermetal product 

1.3 Forging 

1.4 Weldment 

1.5 Stamping 

1.6 Rolled or drawn product (plate, bar, tube, etc.) 

1.7 Extruded product 

1.8 Other material class 

2. Material Type 

2.1 Metallic, ferrous 

2.2 Metallic, nonfeirous 

2.3 Nonmetallic, synthetic 

2.4 Nonmetallic, natural 

3. Material Properties 

3.1 Machinability 

3.2 Hardness 

3.3 Strength 

3.4 Modulus of elasticity 

3.5 Ductility 

3.6 Brittleness 

3.7 Weight 

Specific gravity (density) 
total weight 
weight distribution 

location of center of gravity for unsymmetrical 
or otherwise irregular shapes 

3.8 Magnetic properties 

3.9 Electric resistivity 

3.10 Specific heat 

3.1 1 Thermal conductivity 

4. Part Configuration, Shape, and Size 

4.1 Solid body of the following shapes: 

4.2 Cylindrical 

4.3 Prismatic (bar shaped) 

circular cross section 

polygonal cross section 

structural cross section (angle, tee, etc.) 

short and rigid 

long and flexible 


other shape 

4.5 Spherical 

4.6 Block of the following shapes: 

rectangular sides, square corners 
parallelepiped (skew) 
trapezoidal (in three dimensions) 
full pyramid 
truncated pyramid 

4.7 Hollow body, box, or container, of the previously 
listed shapes 

thick walled 

thin walled 

thin walls with heavier parts (blocks, lumps) 

4.8 Baseplate with uprights 

4.9 Bracket 

4.10 Tube 



thick walled 

thin walled 

with eccentric cavity 

4.11 Irregular shapes (not listed above), and combined 

5. Special Components of Part Configuration 

5.1 Individual components : 

holes splines 

bosses localized cavities 

blocks undercuts 

ribs special surface 
slots and point 

screw threads requirements 

5.2 Number of components listed above 

5.3 Dimensions 

5.4 Locations 


Table 2-1 (Co/it). Part Description Details for Preplanning of Fixture Design 

5.5 Accuracy and tolerances 

5.6 Surface finish (roughness) 

5.7 Surface coatings, if any 

5.8 Other special components, not listed above 

Table 2-2. Classification of Operations for Preplanning of Fixture Design 

/. Machining 

1.1 Bore 

1.2 Broach 

1.3 Drill 

1.4 Mill 

1.5 Plane 

1 .6 Ream 

1.7 Rout 

1.8 Shape 

1.9 Slot 

1.10 Tap 

1.11 Thread 

1.12 Turn 

1.13 Grind 

1.14 Hone 

1.15 Lap 

1.16 Polish 

1.17 Brush 

1.18 ECM (elec tro-chem ical) 

1.19 EDM(electrical discharge) 

1.20 diem mill 

1.21 Manual operations 

1.22 Other 

2. Assembling 



with adhesives 

diffusion bond 








With conventional fasteners 



special types of fasteners 



Blueprints and Specifications 

An examination of blueprints and specifications 
for the part in the light of Table 2-1 will draw the 
designer's attention to the material, size, and weight 
of the part, and any unusual conditions. From the 
material properties he can select the grade of tool 
material to be used and form a first opinion on the 
size and type of fixture required. Table 2-2 is self- 















Swage- fit 




Assemble with a sea) 




3, Inspecting, gaging, measuring 

3.1 Linear dimensions 

3.2 Angular relations 

3.3 Concentricity 

3.4 Planarity 

3.5 Surface quality (roughness) 

3.6 Other inspections 

pressure testing for leakage and rupture 

4. Fix tures for other non-cutting operations 




Cooling after forming of plastic parts 

Surface coatings 


painting (masks) 
Foundry operations 

Number aspects of operations 

5.1 Single operations 

5.2 Operations in prescribed sequence 

5.3 Operations to be performed simultaneously 

explanatory, if and when an operations plan is avail- 
able, but is also useful in cases where the designer 
must do his own operations planning. As the spe- 
cific operation, or operations, have been identified, 
the designer will have a picture of the mechanics of 
the operation, including the distribution, direction, 
and approximate magnitude of cutting forces; their 
character with respect to any tendency for genera- 
tion of shock, vibration, and chatter; and some idea 



Ch. 2 

Table 2-3. Classification of Type of Machines and Other Equipment for 
Preplanning of Fixture Design 

/. Material Removal Machine Took 


Milling machine 



vertical, etc. 


Drill press 


power feed 

multiple spindle, etc. 


Boring machine 


jig borer 

horizontal, etc. 



engine lathe 

face plate lathe 

copying lathe 

turret lathe 

vertical boring mill, etc. 


Linear motion machine tool 



slotting machine 

broaching machine, etc. 


Gear cutting machine 

gear hobbing machine 

gear shaper 

gear grinder, etc. 




surface, etc. 


Abrasive machine tool 

abrasive belt 

abrasive disc, etc. 


Honing machine 


N/C machine tool 

milling machine 

drill press 


tube bender, etc. 


Non-chip cutting machine 

chem mill 


EDM, etc. 



wire brush 

felt or cloth wheel, etc. 

2. Equipment for Manual Work Opera tion g 

2.1 Heat-treating equipment 
high-frequency current, etc. 




Plastic forming equipment 
Surface treatment equipment types 

sand blasting equipment 

shot peening equipment, etc. 
Surface coating equipment 

plating tanks 

painting booths 

drying and baking ovens, etc. 
Foundry equipment 

sand preparation equipment 

molding machines 

core making machines 

mold and core drying and baking ovens 

casting machines, etc. 

Joining Equipment 

3.1 Bonding equipment 

bonding press 
autoclave, etc. 

3.2 Welder 

arc welder 
electron beam 
laser, etc. 



pedestal, etc. 


Stapling machine 


Stitching machine 


Soldering and brazing equipment 


furnace, etc. 


Automatic assembly machines 


Other equipment for joining operations 

4. Inspection Equipment 



comparator « 



operational stage area, etc. 


Inspection fixtures with indicating instruments 

mechanical (dial indicators) 

air gages 

hydraulic pressure gages 

electric meters 

electronic pickups, etc. 

Ch. 2 



about the cutter life and cutter cost to be expected. 
Table 2-3 brings the designer closer to many details. 
A table of this kind is to be used In conjunction 
with lists of the plant's own machine tools with 
tables of their dimensional capacities {table size, ac- 
cessories, horsepower, speed and feed range, etc.) 
and this should essentially conclude the accumula- 
tion of information from sources outside of the fix- 
ture itself. One more aspect should certainly be con- 
sidered, namely, the operator, and the human limita- 
tions imposed, as listed in Table 2-4. 

Each bit of information within this accumulation 
has some bearing one way or another on some point 
within the developing fixture concept. To assist in 
pinpointing the individual subjects within the whole 
and sometimes quite complicated fixture structure, 
Table 2-5 has been prepared, with a list of the basic 
design considerations in the fixture. As the result of 
this development process the preliminary fixture de- 
sign emerges; preliminary because it has not yet 
passed the final test, the economic evaluation. 

Table 2-4. Manipulation and Operator Criteria 

/. Speed considerations 

1 . 1 Lifting, moving, and lowering fixture 


hoisting equipment 

1.2 Loading and unloading of part 

into and out of fixture 

1.3 Clamping of part 
2. Safety of work 

2.1 Locating part correctly in fixture (fool-proof 

2.2 Adjustment of cutter 

J. Operator considerations 

3.1 Fatigue 

3.2 Operational safety 

(accident-proof concept) 

4. Miscellaneous 

4.1 Supply and return of cutting fluid 

4.2 Chip cleaning and disposal 

Table 2-5. General Considerations in Fixture Design 

1. Loading and unloading of part 

1.1 Manual lifting or hoisting 

1.2 Lowering or sliding part into position 

1.3 Unloading to floor 

1.4 Use of magazines, conveyors, and chutes for re- 
ceiving and returning part 

1.5 Speed of motions 

1.6 Ease of motions 

1.7 Safety in manipulations 

2. locating parts in fixture for ready access of cutting tools 

2.1 Concentric to an axis 

2.2 Vertical and horizontal from established surfaces 

2.3 Vertical and horizontal from discrete points 

2.4 Other 

3. damping of part 

3,1 Speed 

Size of clamping forces 


Direction of clamping forces 
Location of clamping forces 

3.5 Manual or power actuation of clamping 

4. Support of part 

4.1 Against clamping pressure 

4.2 Against tool forces 

4.3 Stability of part and avoidance of 
elastic deformation 

5. Positioning cutting tool relative to loaded fixture 

5.1 Rotating ("indexing") 

5.2 Sliding 

5.3 Tilting 

6. Coolant supply and return 

7. Chips 

7.1 Removal of accumulated chips 

7.2 Chip disposal 



Ch. 2 

At this point, a cost estimate and an operational 
estimate should be prepared for the preliminary de- 
sign with the purpose of determining whether the 
savings produced by the fixture in its present design 
will justify its cost. Methods for such economic 
considerations will be discussed further in Chapter 
22, Economics. It may be found desirable to look for 
alternate solutions before the final decision is made. 
A graphical presentation of the design process as 
here described, is shown in Fig. 2-1 

Simultaneously with the economic evaluation, the 
design should be given a comprehensive examination 
for any "hidden flaws," minor, or perhaps major, 
defects which may have been overlooked. This ex- 
amination involves a very large number of details, 
some trivial, some serious. It has been found useful 
to establish a check list for items of this type, and 

to apply the check list to the design at various stages 
and particularly at the time of final approval. 

Check List for Fixture Design 

No list can cover all conditions in every company 
and plant; however, the following list is reasonably 
comprehensive. It also permits the reader to make 
such additions as his special conditions may require. 

The items are listed in their logical sequence. Those 
mentioned first are those which are most likely to 
be overlooked during the initial planning stages. 
Some items are listed in more than one place. 

A check list is the book in a nutshell and has two 

1, It is not supposed to be memorized, but if the 
designer will read it carefully before embarking 

Part Details 





Pri ma r y 
Fixture Design 











Phase I 




Initial Design Concepts 

Fixture Design 

Cost Analysis 
and Evaluation 

Phase II 

Alternate 1 
Fixture Design 

Alternate 2 
Fixture Design 



j , 

Evaluation and Final Decision 
Completion o{ Design, Execution of Shop Drawings 


Fig. 2-1. Outline of the fixture planning process. 

Ch. 2 



on an assignment, it will remain in his mind as a 
constant guide. 

2. It should be applied systematically, point by 
point, whenever a design phase is completed. 

Check List 

1. The Part Drawing 

1.1 Check date and revision references on part blue- 
print and make sure that the print is the latest 
edition and that it is up-to-date with respect to 

1.2 Make at least a cursory check of the part on the 
blueprint; make sure that all views and sections 
are correcdy oriented. 

1.3 Having ascertained the correct shape of the part, 
check all part outlines shown on fixture drawings, 
particularly for correct orientation. 

Z The Shop 

2.1 Make sure that there are no obstructions in the 
shop layout, or around the work station, as well 
as along its access ways, that will prevent or 
otherwise interfere with the transportation of the 
fixture to its station. 

2.2 Investigate whether the work station is equipped 
with the necessary services, such as compressed 
air supply, if needed for activating the fixture. 

2.3 For heavy items (fixture as well as part), a hoist 
should be available. It is advisable to check 
whether it is located properly for the present 

2.4 If lifting equipment is needed and no hoist is 
available at the station, it is recommended that 
the shop department provide other means of lift- 
ing, such as a forklift, 

2.5 Once the fixture is properly located, examine the 
shop layout around the work station to make sure 
that it permits transportation and delivery of 
parts to the work station. 

2.6 The same consideration applies with respect to 
the loading of the part into the fixture. Specific- 
ally, watch out for long parts projecting out of 
the fixture, 

2.7 For a work station that is part of a production 
line, the fixture, when installed, should be cor- 
rectly located and oriented with respect to the 

2.8 For a fixture intended to be used in combination 
with handling equipment for the parts (such as 
gravity feed or mechanical feed hoppers, con- 
veyors, individual hoists, etc.), it is important to 
make sure that the fixture, after completion of 
its design, is also properly incorporated into such 

2.9 For a fixture intended to become a station in a 
transfer line, it is important to make sure that it 
is incorporated into the transfer mechanism. 

3. The Machine Tool 

3.1 Be sure that the fixture will fit the machine tool 
for which it is intended. Check overall dimen- 
sions and the space within which the fixture is to 
be installed (the tooling area). Check dimensions 
of machine tool table, and the dimensions, loca- 
tion, width, and accuracy of T-slots, and com- 
pare with the locating blocks in the fixture base. 
Also check T-slots relative to holes or slots in the 
fixture clamping lugs, specifically for any clear- 
ance required for final adjustment of the fixture 
location relative to the machine tool spindle. 
Inspect the condition of the table slots; if they 
are worn or mutilated they will jeopardize ac- 
curate fixture alignment. 

The fixture must be fully supported and must 
not overhang the edge or end of the table. 
3,2 Investigate the condition of the machine tool to 
make sure that the accuracy is satisfactory for 
the tolerances required in the operation. 

Make sure that the machine is strong and rigid 
enough to carry the weight of the fixture and ab- 
sorb shocks and vibrations from the operation. 
For rotating fixtures (such as on lathe spindles), 
check to be sure that the fixture is balanced with 
part in place. Make sure that breakage of tool, 
part, or fixture does not present a hazard to the 
machine tool or the operator. 
3.3 Checking for interferences. 

The machine must be capable of performing the 
required table traverses and other motions with 
the fixture, the cutter, and the part in place, 
from experience, the depth of throat, that is, 
the distance from center line of machine spindle 
to machine column or machine frame, is a critical 
dimension. With fixture in place, the spindle cen- 
ter line must be brought to coincide with all hole 
center lines. 

The jig, when installed, or when operated, must 
not collide with any projecting part of the ma- 
chine. Beware of screw heads, bosses, accessory 
parts (not always in use), operating handles or 
levers, of the machine tool in all their positions. 
On multiple-spindle machines, where more than 
one fixture or jig is being used, one fixture must 
not interfere with other fixtures. 
3.4 It is a usual and safe practice not to hold a drill 
jig by hand for holes larger than 1 /4-in. (6-mm) 
diameter. In such cases, be sure that stops, rails, 
nests, or fencings can be provided on the drill 
table to prevent rotation of the jig. 

When one jig is used on a multiple-spindle ma- 
chine and moved from spindle to spindle, make 



Ch. 2 

sure that such devices as mentioned above are 
provided under each spindle. 

3.5 For drill jigs to be used with multiple-spindle 
drill heads, it is necessary that the spindles in the 
head can be adjusted to the hole pattern pre- 
scribed by the drill jig. 

3.6 Check the machine too! for required cutting 
speeds and feeds, 

3.7 Provide a tapping attachment for a drill press, 
if needed. 

4. Cutters 

4.1 With respect to general maintenance of cutters, 
it is required that they can be conveniently re- 
moved for resharpening without disturbing any 
setting in the fixture and that they can be ad- 
justed when in place, relative to the fixture. The 
operator or setup man must be able to check the 
correct setting by visual observation. 

4.2 Check for operator safety. Preferably it should 
not be necessary for the operator to have his 
hands close to the cutter; if so, a guard should 
be provided. 

4.3 Checfc for cutter and fixture safety. Make sure 
that the cutter cannot be damaged by contacting 
the fixture. Also, provide that the cutter will not 
cut into clamps, stops, or locators if it is set too 
deeply or overruns its travel. 

Can the cutter {in or out of motion) be con- 
tacted by a clamp when the clamp is being op- 
erated (opened or closed)? 

4.4 Check for interference between cutter and part, 
particularly during loading and unloading. A 
case in point is drilling with multiple-spindle drill 
heads and with the drills on different levels; this 
may require one or a few exceptionally long 
drills which could cause interference. 

Check for other interferences when cutter di- 
mensions (lengths) have been significantly re- 
duced by repeated grinding. Long, slender drills 
running at high speed should be checked for whip- 
ping. A whipping tendency can be eliminated by 
providing a bushing at a high level, with the drill 
running in the bushing. The location of this bush- 
ing must be high enough to allow the jig to be un- 
loaded and reloaded while the drill is running. 
4.5 Check for convenience in cutter operation. The 
fixture should be designed for minimum length 
of cutter travel. In the case of drill jigs, check 
that all holes of the same diameter are drilled to 
their correct depth with one setting of the travel 
stop on the drill spindle. 

A slender drill may enter the drill bushing even 
if the jig is slightly off position. A light jig will 
be moved to correct position by the elasticity of 
the drill. If the jig is heavy, it will not correct its 
position, and drill breakage will be the result. In 
such cases, additional drill guidance is needed. 


Standard cutters should be used to the greatest 
possible extent. They should always be kept 
in stock. 

5. Cutter Setting (other than by drill bushings) 

5.1 Check all cutters individually; determine whether 
they require setting blocks or not. 

5.2 Provide setting blocks where needed. Carefully 
select setting block material (hardened steel, 
carbide, etc.) 

5.3 Supply feeler gages for setting block facings 
where needed. Mark fixture accordingly, at 
proper places. 

5.4 Check to see if a completely and accurately ma- 
chined part can be used as the master for the 
cutter setting. 

6, The Part 

6.1 Check the part for such unusual features as overly 
heavy weight, excessive imbalance, and long pro- 
jecting ends, and provide for required supports. 
Hard and abrasive part material may require car- 
bide faced locating and supporting elements. 
Very soft part material, or parts with prema- 
chined surfaces may require protection against 
scratching or being otherwise marred by the pres- 
sure from clamps and support points. Very thin- 
walled, or otherwise nonrigid shapes may require 
special attention (auxiliary supports, or the like) 
against tending or being otherwise distorted by 
the clamping and operating forces. Clamps may 
be provided with leather, rubber, fiber, copper, 
or brass facings. Clamps may be hand-operated 
with knurled head screws to prevent application 
of excessive torque. 

6.2 Check for the effect of variations in the shape 
and dimensions of the part. Allow clearance in 
the jig for normal dimensional variations {forg- 
ings, castings), and for any protruding elements 
(bosses, ribs, lugs, etc.), that are integral parts of 
the normal geometry. 

Look for mismatched castings; preferably, have 
all locating points on one side of parting line. 
Look for flash on forgings; do not place locators 
in line of flash. 

In case of serious locating problems, consult 
with the engineering department, for the possi- 
bility of permanent modification of part design 
to facilitate production, or temporary modifica- 
tion, such as the addition of lugs or flanges for 
locating and clamping purposes, to be removed 
by subsequent machining. 

In drill jigs of simple type, the gathering of all 
drill bushings in one jig plate will ensure correct 
relative position of all holes. 
6.3 Some surfaces on castings and drop forgings have 
draft, ranging from 1/2 to 7 degrees, even if the 
part drawing may show them as parallel to each 
other and perpendicular to the base plane. 

Ch. 2 



7. Locating the Part (Rough Parts) 

7.1 Check the part for best possible surfaces oi points 
for locating. Criteria for evaluation of such items 
are static stability and compatibility with, and 
good definition of, the surfaces to be machined; 
preferably, such points or surfaces as are dimen- 
sioned directly from the machined surface(s). 

7.2 Wherever possible, provide viewing holes in fix- 
ture for visual check on locating, or profile plates 
to assist in locating and chip cleaning. 

7.3 For rough surfaces, buttons are better than flat 
blocks. They should be hardened, except for 
very short runs. 

7.4 Space locating points as widely as possible, par- 
ticularly on rough surfaces use three locating 
points. If a fourth (or more) point is needed for 
stability, make it adjustable or a jack type. 

7.5 Centralizing devices, such as V-blocks for cylin- 
ders, are insensitive to diameter variation with 
respect to locating the plane of symmetry, but 
not to the perpendicular location (the height) of 
the center line. 

7.6 The stacy ng of several parts in one fixture may 
result in an accumulation of error. In such cases, 
estimate the accumulated error; if unacceptable, 
provide individual locators for each part. 

& Locating the Part (Premachined Parts) 

8.1 Check previously machined surfaces for suitabil- 
ity as locating surfaces. The basic condition is 
that their tolerances must be fine enough for the 
accuracy required in the following operations. 
All subsequent locating and machining operations 
should be based on the same locating points and/ 
or surfaces, 

8.2 If a curved surface blends with a flat surface, 
locate from the flat surface for the curved surface. 

8.3 Locating surfaces in the fixture should be kept as 
small as possible, should be relieved to allow for 
any burr from previous machining, should be kept 
within the outline of previous machined surfaces, 
should be elevated from the fixture base so as not 
to be buried in chips, and should be easily 

8.4 Locating elements are subject to wear, even 
abuse, and should be easily replaceable. 

8.5 A locating pin, to match a drilled hole, should 
have a conical lop for easy catch. If locating 
from two drilled holes, make at least one pin a 
diamond pin. Make them of unequal length foi 
easy catch. 

Locating pins for eounterbores should locate 
on one diameter only. 

Provide clearance at the base of all locating 

9. Clamping 

9.1 Evaluate the operation in terms of part weight 
and cutting forces. It may be light, medium, or 
heavy. In the case of a light operation, it may be 
possible that the part can be held in the fixture 
by virtue of its shape or weight only, without 
actual clamping devices. 

9.2 Check the support and clamping system for sta- 
bility and strength, specifically the following: 
Supporting and clamping points should be se- 
lected as-wide apart as possible. The part should 
be supported directly below the clamping points 
with solid metal in-between, and close to the 
points of action of the cutting forces. 

The cutting forces should act to hold the part 
in position; avoid a design where the cutting force 
acts to lift, tip, or tilt the part. Do not rely on 
friction to resist the cutting force. Preferably, 
the cutting force should be directed against the 
supporting points. There are a few special cases 
where it is correct to have the cutting force act- 
ing against the clamps, Note again that these 
cases are few and special. The cutting and clamp- 
ing forces should not act to distort the part or 
the fixture. This must be prevented by providing 
adequate support points. The cutting forces 
should not act to upset, tilt, twist, or otherwise 
displace the fixture on the machine table. 

For milling fixtures, check the previous points 
for the two cases of up-milling and down-milling 
(also known as climb-milling). Check force and 
stress analysis to make sure that the fixture and 
the clamps have ample strength and rigidity to 
withstand all loads. 

Make sure that clamps cannot be loosened ac- 
cidentally by centrifugal forces, shock, vibration, 
and chatter, 

9.3 For parts of varying dimensions (castings, forg- 
ings) check the clamp and equalizers to see that 
they have enough range to cover all dimensional 
variations within tolerances. 

9.4 The clamp must not In any position, open or 
close, interfere or collide with any part of the 

9.5 Clamps must be so arranged that they cannot 
damage the machine or the operator even if they 
are forcefully opened or accidentally dropped. 
They must not generate strain in the machine 
tool structure even if excessive clamping force is 
applied. All forces from clamping must be re- 
tained inside the structure of the fixture. 

9.6 Investigate possibilities for activating or driving 
clamping devices from the machine tool. Ex- 
amples: air-operated clamp with its control valve 
connected to the start or feed lever on the ma- 
chine tool. Hydraulic clamp, supplied with oil 



Ch. 2 

from machine tool's hydraulic system. Mech- 
anized clamp, actuated by some feed motion in 
the machine tool, 

9.7 For N/C machine tools, investigate the use of air 
or hydrauljcally operated clamping mechanisms, 
activated by the N/C program. 

9.8 It is preferred to use floating clamps on cast, or 
otherwise irregular, surfaces. 

9.9 Evaluate the various aspects of the clamping op- 
eration, whether it is a quick and easy operation, 
or requires hard work by the operator, which 
should be avoided. Check all provisions that fa- 
cilitate the clamping operation, such as: Have at) 
clamps accessible from one side (operator's side) 
of fixture, use only one standard size of bolt 
heads, and only one spanner wrench. Better stilt, 
if possible, operate by hand without a wrench. 
Ideally, combine ail clampings so that they are 
operated simultaneously by one clamping lever. 

Avoid loose pieces; tie them to the fixture by 
pins, hinges, or chains. 

Use compression springs under straps to hold 

them up when the bolt is unscrewed, and provide 

elongated bolt holes so that they can be retracted 

by a short, straight pull. Use stops to prevent ro- 

. fating w he n 1 o osened an d (igh tene d . 

On large and heavy clamping straps, use a handle 
for convenient lifting. 

Look for two-hand operations. Examples: with 
a suitable handle on the fixture, the fixture is 
held by one hand while the clamp is aotivated by 
the other hand. One hand clamps while the other 
hand engages the feed lever. 

Use toggle clamps for quick action. Use com- 
pressed air or hydraulics for quick action and/or 
large clamping forces. 

10. Loading and Unloading 

10.1 Many little details make a big difference with ' 
respect to loading and unloading, such as: Lo- 
cating points should be clearly visible, if possible. 
Provide sufficient clearance for the part in alt po- 
sitions during loading and unloading. Check 
clearance relative to the fixture walls, the locating 
and centering devices, and the clamps. Allow 
plenty of room for the operator's hands. Heavy 
parts should be located end for end; one end is 
supported while the other end is being located. 

10.2 Small details in the design of locating pins may 
be significant. 

Make them as short and as small as possible. 
With two locating pins, make one pin longer than 
the other so the part is located on one pin at a 
time. Use the diamond-shaped cross section, 
where appropriate. Make ends of pins pointed 
and rounded for easy catch. 

10.3 For heavy parts to be located in previously 
drilled holes, use disappearing locating pins so 
the part can slide over them into position. 

10.4 Provide comfortable handles of ample size for a 
good grip on hand-operated locators. Knurled 
handles, particularly small ones, are considerably 
less comfortable to operate than star-shaped 

Combine movable jacks, plungers, and other 
locators so that they are operated by one handle 
and, if possible, are locked automatically by the 
clamping operation. 

Place all operational devices on operator's side 
of the fixture. 

10.5 Provide additional stops and guides so that the 
part can enter the fixture in one position only- 
the correct one. Allow for burrs from previous 

10.6 Wherever possible, pre-position part for easy load- 
ing during machining cycle on the previous part. 
Fixtures may be built with duplicate space for 
parts so that one space is unloaded and reloaded, 
while the part in the other space is being machined. 

Or: the finished part may be removed by one 
hand, while the new part is inserted by the other 

Or: The finished part is ejected by inserting the 
new part. 

10.7 The part may be unloaded by means of an ejector 
which may be automatically operated when the 
clamps are loosened. 

11. Drill Bushings 

11.1 Check each drilling operation with respect to the 
necessity of a bushing. Many operations, in par- 
ticular, second operations (countersinking, ream- 
ing, tapping), can be guided by a previously pro- 
vided hole and do not need a bushing. 

11.2 Straight bushings (i.e., without flange) are pre- 
ferred because of their simplicity and low cost; 
however, they are totally dependent on their press 
fit in the plate. If the press fit fails, they may be 
pushed down by the drill. All straight bushings 
should be checked for this eventuality and if such 
displacement can be harmful, they should be re- 
placed by flanged bushings. 

11.3 Flanges on bushings should protrude above accu- 
mulation of chips and cutting fluid. Wherever 
possible, use flange top as a depth stop for the 
drilling operation by providing a hardened collar 
on the cutter. 

1 1.4 Bushings must have a chamfer or rounded edge to 
catch and guide the drill point. 

11.5 Check length of bushings; they must be long 
enough to give adequate support, but bushings 
that are too long cause excessive friction and 
wear. Accurate work requires bushings to be car- 
ried near the surface of the part; the same applies 
to contoured and inclined surfaces where the cut- 
ter cannot enter squarely. Cut end of bushings 
with a plane cut, not a contoured cut. 

Ch. 2 



1 1 .6 For subsequent operations in the same hole, slip 
bushings are used. Check to see that liner bush- 
ings aie hardened. Slip bushings may be omitted 
by using a stepped drill for drilling and counter- 
boring in one operation. Let the large diameter 
enter the bushing before cutting starts. A hinged 
bushing plate can also replace slip bushings. 

11.7 It is useful to have certain markings on drill jigs 
and on some bushings. The drill jig should be 
marked with drill size adjacent to bushings. Slip 
bushings should be marked "drill" or "ream," as 

11.8 Slip bu shi ngs r equ ire spec ial at ten tion . First, they 
should only be used when absolutely necessary 
(see 1 1 .6). They should be effectively locked in 
place. Flanges should be large and fluted (prefer- 
able to knurled) for easy gripping, with room 
underneath for the fingertips, and a stop against 
turning in the lift-out position, and perhaps even 
with a handle. 

12. Drill Jigs 

12.1 A drill jig for one hole only may advantageously 
be clamped in position centered under the drill 
spindle. In other cases, the jig or the drill spindle 
(a radial drill) has to be moved from hole to hole. 
The choice depends on size and weight of the jig. 

12.2 Only small jigs may be held by hand. For hole 
sizes larger than 1 /4-inch (6 rnm) diameter, provi- 
sion must be made to resist the drilling torque. 
Small jigs may be provided with a handle for this 
purpose; larger jigs require positive stops. 

12.3 Provide feet under the jig, large enough to span 
the table slots, and high enough to prevent drill- 
ing into the table. Space the feet so that all cut- 
ting forces act inside the feet. 

1 2.4 All bushings should be clearly visible to the op- 
erator when jig is positioned. 

12.5 For holes in a circular pattern, the jig may be 
mounted on a rotary table. 

12.6 For holes in several directions, the jig may be 
rolled over (tumble jig). This motion may be 
facilitated by providing rockers (cradle fashion), 
mounting jig on trunnions, or resting it in angle 

12.7 The most advanced device for the same purpose 
is to provide indexing for the jig. Index pins 
must move in and out easily, engage quickly and 
accurately, be locked against loosening when en- 
gaged, and unlocked and withdrawn preferably 
by one movement. 

12.8 A handwheel may be useful in operating an in- 
dexing fixture. 

IS, Fixtures in General 

13.1 Look for and avoid, difficult or awkward setup 
conditions which could be caused by heavy 

weight, uneven weight distribution, holddown 
bolts in difficult locations, etc. 

13.2 The wrenches again! Preferably, one size only for 
setup and one size only for operating. 

13.3 Unusual accuracy requirements may call for the 
use of dial indicators. Provide brackets or other 
means for mounting them. Fixture may be de- 
signed with adjustment devices to compensate for 
machine tool misalignment. 

Precision locating from previously bored holes 
may require expanding plugs to eliminate effect 
of diameter tolerances, 

1 3.4 Boring fixtures {not the same as drill jigs) have a 
special sequence rule: locate part from the smal- 
lest bore; reason: this avoids an eccentric cut 
with the smallest boting bar. 

Boring fixtures may advantageously use hall- 
bearing mounted pilot bushings for small-diameter 
boring bars running at high speed. 

13.5 Use stock castings or other stock material for the 
fixture body, wherever possible. 

13.6 For milling operations, use a standard vise with 
special jaws, wherever possible. 

13.7 Keep the design low. 

13.8 Precision components within the fixture should 
be fastened by means of screws and located by 
means of dowel pins or keys, 

14. The Chips 

14.1 Chips may be continuous (smooth and shiny) or 
discontinuous (short sections, integrated into fi- 
nite lengths, easily broken), both stringy, and 
produced from steel and aluminum ; or crumbling 
(small pieces), even powdery, produced from cast 
iron and bronze. Establish the chip type that is 
produced when machining the part. 

14.2 Crumbling chips require space to escape between 
surface of part and end of drill bushing; stringy 
chips require bushing carried close to surface to 
guide chips up through bushing. 

14.3 Cutters should have ample space in flutes to allow 
chips to form; flutes should be carried well above 
surface of fixture to allow chips to escape. Pilot 
ends on tools should have grooves or flutes for 
chips, to prevent binding in bushings. 

14.4 Chips tend to collect in the bottom of the fix- 
ture. Avoid forming corners and pockets that 
can collect chips. Provide openings and inclined 
paths for chip escape and chip cleaning. Cutting 
fluid may be directed so as to assist in chip 

Lift surfaces of locators and supports above 
possible chip accumulations. Keep such surfaces 
small in area and provide for cleaning. 

V-blocks and other locators with reentrant sur- 
faces should have clearance for chips and burrs. 



Ch. 2 





Prevent chips from entangling in clamp lifting 
springs, Protect movable parts such as plungers, 
jacks, index plates, etc., from chips. If necessary, 
provide shielding. 

If necessary, provide chip cleaning equipment, 
ranging from rakes, forks, and scrapers, to me- 
chanical chip conveyors. Be reluctant to use 
compressed air; while highly effective on crum- 
bling and powdery chips, it contaminates the 
shop atmosphere with abrasive dust. 

Protect rotating machine parts from entangling 
long chips. 

Chips on table may jeopardize position if they get 
under fixture feet. Keep area of feet small (nar- 
row, angular, or T-shape). Non-integral feet 
should have press fit or be fastened by screws 
from the top, avoiding chip pockets in bearing 

15. Cutting Fluid 

15.1 The cutting fluid must reach the edge of the cut- 
ter. If necessary, provide channels or guides for 
this purpose. 

15.2 Provide channels, guides, and guards to prevent 
cutting fluid from running to waste, from being 
spilled on the machine or the floor, and from 
hitting the operator. 

15.3 Utilize the flow to wash chips away. 

15.4 Cutting fluid may serve as lubricant for movable 
fixture parts. Provide necessary holes for this 
purpose, as required. 

16. Safety 

16.1 Make a general review of all manipulations and 
operations to be performed on the part to ensure 
that they do not present any hazard. 

16.2 Make a similar cheek with respect to the cutter; 
in particular, if the part should be incorrectly 
loaded, will this cause damage to the cutter? Take 
a last and critical look at the' dimensions of the 
cutter and its support {arbor, chuck, toolholder). 

16.3 Check fixture and part for visibility at all times 
(loading, positioning, clamping, cutter approach, 
cutting, possible overrun, cutting fluid, chips, un- 
clamping, and unloading). 

16.4 Check fixture design for even the most trivial 
hazards: sharp edges and corners, projecting 
screw heads, levers and handles, finger clearance 
around and under handles, grips, slip bushings, etc. 

16.5 Make sure that hinged and otherwise movable and 
heavy parts (leaf plates, clamps, cover plates) can- 
not accidentally fall upon operator's fingers; tike- 
wise, check motion or air-operated clamps. 

16.6 In case of accidental failure of a clamp, can the 
part be thrown out? Specifically, in case air 
pressure fails. 

16.7 See that operator is protected against flying chips ■ 
and any splashing cutting fluid. 

16.8 Consider the use of notices of caution on the 

17. Inspection of Pan 

17.1 It is not usually necessary to measure and inspect 
the part while it is in the fixture; however, when 
that is required, consider the following points. 

17.2 Provide necessary space for cleaning of surfaces, 
insertion of gages or instruments, and the opera- 
tor's hand; for clearance against the cutter, datum 
surfaces, measuring blocks, etc., as required. Pro- 
vide visibility of measured surfaces and measuring 
instruments, and beware of burrs. 

17.3 An unusual, but not impossible, case occurs when 
the fixture containing the part must be removed 
from the machine and brought to the inspection 
device. This calls for close cooperation both in 
the design of the fixture and of the inspection 

18. Manufacture, Maintenance, Handling, and 
Storage of Fixtures 

18. 1 With the design finalized, check the cost analysis. 
Beware of the following two cases: (a) the cost 
is too high for the production volume anticipated 
(b) the production volume is high enough to jus- 
tify a more sophisticated, more efficient, and 
more expensive fixture. 

18.2 Check for maximum utilization of standard and 
commercial parts. This applies not only to drill 
bushings, but also: handles, stops, supports, 
clamps, feet and numerous other parts, including 
the complete so-called "universal fixture." Check 
manufactured parts for use of stock sizes of ma- 
terials. Finally, check availability of all such 

1 8.3 Check your own toolroom facilities for manufac- 
turing capability with respect to this fixture, in- 
cluding its prescribed tolerances. 

1 8.4 Parts to be heat treated should have provision for 
suspension; if necessary, drill a small hole for that 

1 8.5 Check tor lubrication possibility of all movable 
parts, for correct material selection and specifica- 
tion of heat treatments, for hard surfaces on all 
wearing parts, and for easy removal and replace- 
ment of parts subject to wear or accidental dam- 
age. Be certain that replacements can be made 
without interference with other fixture elements. 
Make sure of access to the "inner end" of parts 
with a press fit. 

18.6 Provide aids for lifting, such as; lugs, eyebolts, 
hooks, chain slots on heavy fixtures, and handles 
on heavy loose parts. 

Ch. 2 



18.7 Secure small loose parts and hand toots against 
loss in storage (chains, keeper screws, etc.); do 
not rely on tape. 

18.8 For storage, provide protective cover, case, or a 
box for weak and delicate parts, precision and 
polished surfaces, or the complete fixture. 

18.9 When in storage, the fixture should rest in a well 
balanced and stable position. If necessary, pro- 
vide a suitably profiled base plate or crate for 
this purpose. 

18.10 Provide all necessary identification marks, in- 
cluding marks on any hose items. 

18.11 Check design drawings for correct sections, pro- 
jections, and views, and a clear description of all 
functions, for complete dimensioning, complete 
information notes, and clear, correct, and com- 
plete title block. Do not forget tool identification 

18.12 During this final check, look once again for the 

following details: Strength, rigidity, simplicity, 
safety, under operating conditions as well as 
under accidental overloading. 

Sufficient mass to absorb shock and vibration. 

Tolerances, as liberal as possible for the purpose. 

Support surfaces, datum surfaces, and grinding 
clearances for the toolmaking. Provision for 
alignment of precision press-fitted precision parts. 
Avoid blind holes. Do not forget countersinks 
where desirable. Avoid square and polygonal 
holes (for plungers and similar moving parts). 
Provide breathing holes on bores for plungers. 
Dowel pins, precision pins, locating keys for parts 
requiring repeated accurate positioning, spaced 
as widely as possible. 

18.13 For precision operation, the fixture may require 
a scraped fit to the machine table. 

18.14 When future changes in part design can be an- 
ticipated, provision should be made in the fix- 
ture design. 


The Fixture Design Procedure 

The process of fixture design differs drastically 
from the design process usually applied to a machine 
part. The conventional method of machine design 
consists, in broad terms, of first making an approxi- 
mate sketch of the outline of the part, with axis 
lines and significant points indicated, and assisted 
by appropriate sections, The loads are then applied, 
bending moments and stresses are calculated, and 
the dimensions are altered where necessary until the 
stress analysis is satisfactory. 

Steps in the Fixture Design Process 

In fixture design, the outline of the fixture is 
about the last step in the process. The sequence 
is locating, clamping, supporting, applying cutter 
guides, and, finally, drawing the fixture outline as 
the envelope that combines all the previously drawn 

In the practical design process, however, one op- 
erates continuously with the part, and it is recom- 
mended that the part outline, or appropriate sec- 
tions in the fixture sketches, should be drawn either 
in thin lines or, preferably, in colored lines. 

Locating and Degrees of Freedom 

Locating the part is, at this stage, a geometrical 
concept; the acting forces (weight, clamping pres- 
sures, and cutting forces) are not taken into con- 
sideration with respect to their magnitude, but only 
as to their direction, so as to ensure that the part is 
located in a position of static stability. 

A "small" particle (a point), when unsupported, 
has three degrees of freedom in space. It can move 
in any direction, but any motion it may perform is 
fully defined by its components in three directions; 
in mathematical language, by three coordinates. If 
these three motion components are arrested, the 
particle cannot move; it has been deprived of its 
three degrees of freedom. 

A "body" consisting of several, or many, particles, 
can move as a particle, but it can also rotate; it has 
three axes of rotation, and as any rotary motion of 
the body can be described by three rotation com- 
ponents around three axes, the three linear motions 
plus three rotations make six degrees of freedom. 
The six degrees of freedom can be made up by com- 
ponents other than those just described, and several 
other such component sets, at various places in the 
following, will be applied to the problem of locating 
a part in a fixture. 

As a simple and fully realistic example, consider a 
rectangular block with unmachined rough sides, Le., 
a casting or a forging. Locate the bottom surface on 
three points not in a straight line (see Fig. 3- 1 a) and 
assume holddown forces to so act on the block, that 
it cannot be lifted off. These three points prevent 
motion in the vertical direction and also prevent 
rotation around a longitudinal and a crosswise axis. 
In other words, they have deprived the block of 
three degrees of freedom. The block can stiU slide 
in two directions in the plane defined by the! three 
points (two degrees of freedom) and can rotate 
around a vertical axis (the third degree of freedom). 
Now add (see Fig. 3-1 b) two locating points against 
one of the vertical sides, not in the same vertical 
line, and again with holddown forces. This prevents 
motion in the crosswise direction and also rotation 
around the vertical axes, and thus deprives the block 
of two degrees of freedom. It has one degree of free- 
dom left, namely, motion lengthwise. Finally, apply 
one locating point (with holddown force) against the 
end (see Fig. 3-1 c); this eliminates the sixth degree of 
freedom and the block is now fully located. The 
addition of a fourth point at the bottom surface 
(see Fig. 3-1 d) would theoretically make the system 
redundant, but is used under certain conditions to 
improve the stability, as explained in Chapter 4, 
Locating Principles. 


Ch. 3 



Fig. 3-1 . The principle of locating a part by single locating 
points, a, b, c. Application of three points on the 
bottom surface, d. Application of four points on 
the bottom surface. 

In actual fixture design, the very first step is to 
deprive the part of its six degrees of freedom. To 
apply six individual locating points, as described 
above, is perfectly possible, but the six degrees of 
freedom can be eliminated in many other ways, 
each of which is geometrically equivalent to the 
six locating points. 

Using the Clamping Elements 

The hold- down forces referred to above were 
imaginary forces. In actual design, the next step 
would be to apply real clamping elements such as 
bolts, straps, cams, etc., in such places that the part 
is held firmly against the locating elements, not only 
as it is being located, but also at the time that the 
cutting forces become active. The number of clamp- 
ing elements used are not necessarily equal to the 
number of locating points. For example, in the il- 
lustration shown, one vertical clamp over the center 
of the block (arrow C, Fig, 3-1 a) would suffice to 
take care of the three locating points; one additional 
clamp against the center of one side (arrow C, Fig. 
3-1 b) would take care of two points; in fact, one 
clamp (arrow C, Fig. 3-lc) acting on a corner and 
directed along the diagonal, would take care of all 
six locating points. 

This is, however, a purely theoretical concept. 
One important rule at this point is that a clamping 
force must be applied as directly as possible, and 
without causing any elastic deformation or "spring- 
ing" of the part. In case the block in Fig. 3-1 is of 
solid metal, it may well be clamped with two or 
three clamps only; but if it should be hollow, it 
may be necessary to allow for additional clamps so 
that each clamping pressure is transmitted through 
solid material (preferably) to its reaction point. 

Providing Support 

The next step involves "support." The term, as 
used here, is slightly misleading because some sup- 
port has already been provided by the locating 
points. However, the locators have only provided 
sufficient support to secure the geometrical stability 
of the part, and this may not be sufficient to absorb 
all acting loads without causing elastic deformation 
of the part. The supports to be supplied in this de- 
sign step must be sufficient in number and strength 
to absorb all acting loads. On the other hand, they 
must not interfere with the locating of the part, as 
already established. They are, therefore, made ad- 
justable and brought to bear against the part with- 
out significant pressure and without producing any 
geometrical redundancy. 



Ch. 3 

Cutter Guidance 

The following step is to provide guidance for the 
cutter. For drill jigs, this means to draw the drill 
bushings in their proper place in the design. For 
other types of fixtures (milling, planing, etc), the 
tool guides are actually points for positioning the 
tool prior to the start of the cut. 

Completing the Body 

Until now, all the elements are drawn "floating 
in the air," so to speak; the last remaining design 
step is to draw a jig or fixture body that carries 
all the individual elements and has enough strength 
and rigidity to hold them in their proper places 
under load. 

The fixture body must also fulfill a number of 
other conditions-it must first accommodate the 
part, have clearance for loading and unloading, and 
for chips; it should have feet or some other sup- 
porting surfaces to carry it on the machine table; 
have locating elements for aligning it with the ma- 
chine spindle, and have an adequate number of lugs 
for holddown bolts. 

Categories of Fixture Materials 

The following categories of materials are used in 

Steel, not formally specified 

Steel, specified by the SAE and A IS I classifica- 
tions and standards 

Tool steel 

Cast iron 

Aluminum and magnesium 

Sintered tungsten carbide 


Other nonmetallic materials 
Detailed analyses, mechanical properties, and heat 
treatment instructions of these materials are, as a 
general rule, not listed in the following sections. 
The data for standardized materials are readily avail- 
able in reference books, i.e., Machinery's Handbook 1 . 
Data for materials that are not covered by standards, 
are available from the manufacturers. 


Steel, Not Specified 

Much steel material is used without reference to 
any standard. It is unalloyed low carbon steel (car- 

bon content from 0.18 to 0.25 percent, usually 
around 0.20 percent). It is available as "hot rolled" 
and as "cold rolled," or otherwise cold finished. 
Hot rolled plate is also known as "boiler plate," 
which is widely used for fixture bodies, welded and 
non-welded, and for a variety of parts that do not 
require hardened surface or superior strength. 

Standard Steels 

A long range of steels are standardized and iden- 
tified by the prefix SAE (Society of Automotive 
Engineers) or AISI (American Iron and Steel Insti- 
tute), followed by a four-digit number. The last 
two digits indicate the carbon content (SAE 1020 
contains from 0.18 to 0.23 percent carbon). The 
first digit indicates the class to which the steel be- 
longs. The classes are: 


Types of Steel 


plain carbon steels 


free cutting (carbon) steels 


manganese steels 


nickel steels 

3 xxx 

nickel-chromium steels 

4 xxx 

molybdenum steels 


chromium steels 


chromium-vanadium steels 


tungsten steels 


nickel-chromium-molybdenum steels 


silicon-manganese steels 

Erik Oberg and F, D, Jones, Machinery's Handbook 
(New York: Industrial Press Inc., 1971.) 19th ed., pp. 

The large class of carbon steels, 1 Oxx, can be sub- 
divided into the following groups: 

1015 and below, are highly ductile and are 
used for press work, but not for fixtures. 

The low carbon group is numbered 1016, 1017, 
1018, 1019, 1020, 1021, 1022, 1023, 1024, 
1025, 1026, 1027, and 1030. They are avail- 
able hot rolled and cold finished; and round 
bars are also available with a ground finish. The 
steels are readily weldableand easy to machine. 
The grades up to and including 1024 are the 
principal carburizmg or case hardening grades. 
The grades from 1022 and up, when car- 
burized, are oil hardening, the lower grades 
are water hardening. The grades from 1025 
and up, while not usually regarded as carburiz- 
ing types, are sometimes used in this manner 
for large sections or where a greater core hard- 
ness is required. 

The medium carbon group is numbered 1030, 
1033, 1034, 1035, 1036, 1038, 1039, 1040, 

Ch. 3 



1041, 1042, 1043, 1045, 1046, 1049, 1050, 
and 1052. They are available hot rolled and 
most of them are also available cold finished. 
They are readily machinable and have higher 
strength than the previous group although 
they still retain satisfactory ductility and 
toughness. Their mechanical properties are 
further improved by heat treatment. Steels 
with less than 0.40 percent carbon cannot be 
hardened above 45 Rockwell C. 
The high carbon group is numbered 1055, 
1060. 1062, 1064, 1065, 1066, 1070, 1074, 
1078, 1080, 1085, 1086, 1090, and 1095. 
Available hot rolled, most of them are also to 
be had as drill rod; that is, round bars, ground 
to close tolerances. They have higher strength 
and hardness than the previous groups, have 
satisfactory toughness, but low ductility and 
are used where higher strength and toughness 
or greater wear resistance is required. They 
are, to some extent, hardenable, but the hard- 
ness obtained depends largely on the rate of 
cooling during the quenching operation. The 
high cooling rate (critical cooling rate) required 
for maximum hardness is only present in the 
surface layer where the metal is in direct con- 
tact with the cooling medium; in the interior 
of the material, the cooling rate is less than 
critical, and the full hardness is therefore con- 
fined to a shallow surface layer. This is known 
as the "mass effect." In heavier sections, the 
mass effect is so dominating that even the sur- 
face cannot attain the theoretical full hardness 
of 62-64 Rockwell C. 
A large number of stainless steels are standardized, 
but are not included in the list above. Stainless 
steels arecorrosion resistant and most have excellent 
mechanical properties. Due to their higher cost and 
lower machinability, they are not used to any signi- 
ficant extent in fixture construction. However, some 
fixture components are now commercially available 
in stainless steel. Stainless steel is also used for pur- 
poses where its lack of magnetic properties is of 
value, such as for separating elements in magnetic 
chucks and for backing bars in welding fixtures. 
Alloy steels, in general, have greater toughness 
than carbon steels of comparable strength. Also, 
the wear resistance is greater than for a carbon steel 
of the same carbon content. They have better har- 
denability than carbon steels, which works out in 
two ways: ( 1 ) They harden in depth, not just in the 
surface, and for the hardening they require a less 
severe heat treatment than carbon steels; (2) They 
distort less, therefore, during heat treatment, and 

have better dimensional stability after heat treat- 
ment because they contain less residual stresses. 
The cost is higher, however, and alloy steels are only 
selected where a carbon steel cannot be used. 

Tool Steels 

All tool steels normally used in jig and fixture 
construction have a high carbon content. Some of 
them are carbon steels, but most of them are alloy 
steels. They attain the highest possible hardness and 
wear resistance when quenched and can be tempered 
back to satisfactory toughness without undue loss 
of hardness. They are not as readily machinable as 
the carbon and low alloy steels, so that cutting 
speeds are lower and the machined surface is not as 
smooth, Where accuracy and dimensional stability 
are important, it is necessary to pre-machine and 
stress anneal or normalize the part for stress relief 
and then machine it to final dimensions. 

Tool steels differ widely with respect to distortion 
during heat treatment. For fixture parts it is desir- 
able to keep distortion at a minimum and this is a 
major consideration in the selection of the type of 
tool steel to be used. Practically all fixture require- 
ments can be met with the selection of tool steels 
listed in Table 3-1 , comprising two water hardening 
steels (W-), three oil hardening steels (0-) and SAE 
52100 which is not really a tool steel, but very 
suitable for some fixture purposes. High-speed 
steels are not included in the list because they are 
seldom used in fixtures. 

Table 3-1. Selected Tool Steels 
for Fixture Components 


Type of Steel 








SAE 52100 






















Steel Material Selection 

Shop language has its own non-standardized no- 
menclature for various classes of steel. Machine 
steel is hot rolled, low carbon steel, as opposed to 
cold rolled steel. Low carbon tool steel is carton 
steel of less than 0.60 percent carbon; the lower 



Ch. 3 

limit is undefined. High carbon tool steel is carbon 
steel of 0.60 percent carbon, and higher. Low alloy 
tool steel means the chromium-molybdenum and 
nickel- chromium-molybdenum steels in the41xx and 
43xx class. It is actually a misnomer because the car- 
bon content in these steels never even reaches 0.55 

Tool steel is usually purchased hot rolled and an- 
nealed and is therefore decarburizcd in the surface. 
The machining allowance must be sufficient to en- 
sure that all decarburized material is removed. Mini- 
mum machining allowances are as follows: 


on Diameter 





on Diameter 

1/2 or less 
over 1/2 

to 1 1/4 
over 11/4 

to 2 1/2 
over 2 1/2 

to 5 

13 or less 

13 to 32 

32 to 63 
63 to 127 




1 1/2 



5 and over 

127 and 



The surfaces of low carbon steel parts that require 
case hardening must be fully carburized to a depth 
of at least 1/16 inch. 

Cold finished material is preferred to hot rolled 
wherever possible. It saves machining when it is 
used in the fixture in standard dimensions and it has 
a somewhat higher tensile and yield strength and 
surface hardness than hot rolled material because it 
is work hardened. It also produces a smoother sur- 
face when machined, however, since it contains 
higher residual stresses than hot rolled material it 
may warp if it is asymmetrically machined. For 
this reason, hot rolled material is preferred for parts 
that are to be finished by grinding, or machined on 
one side only. 

Selection of Proper Steels for Parts 

Small and medium-size drill bushings are made of 
oil hardening tool steels, grades 02 and 03, or of 
52100 steel because of their low distortion in heat 
treatment. Larger drill bushings are made of 8620 
steel, carburized and case hardened. Very large 
bushings are made of low alloy steels in the 41xx 
group. Cheap bushings are made of 1 060 or 1 065 
carbon steel. Locating parts, such as buttons, pins, 
and pads, that require a high hardness and are finish 
ground after heat treatment, are made of water har- 

dening steel W] or W2, quenched in brine and tem- 
pered at 300 to 37 5F (149 to 191C). Cams that 
cannot be contour ground after heat treatment and, 
therefore, must be machined or hand filed to final 
dimensions are made of oil hardening tool steel, 
grades 02 or 06. 

Chuck and vise jaws that can be ground after 
hardening are made of grades Wl or W2; if finish 
grinding is not possible, they are made from grades 
02 or 06. 

Miscellaneous parts of small and medium size that 
do not carry any significant load but require com- 
plete or partial case hardening for wear resistance 
are made of 1018, 1020, 1133, or 1144 carbon 
steel. Surfaces that are exposed to light wear only, 
can be case hardened by cyanizing and do not need 
subsequent grinding unless a high degree of ac- 
curacy is required. 

Highly stressed parts, such as arbors or mandrels, 
boring bars, highly loaded clamps, and fixture de- 
tails are made from alloy steels with medium carbon 
content such as 2340, 2345, 3140, 3145, 4140, 
5140, 6140, and 6145 with or without heat treat- 
ment, depending on the stress level during opera- 
tion. For particularly heavy duty, oil hardening 
06 tool steel is used. 

Collets and expanding arbors are particularly crit- 
ical because they require hard gripping surfaces com- 
bined with resilience. They are made from alloy 
steels with low carbon content, such as 2312, 2320, 
3115, 3120, 4615, 6115 and 6120 or from oil har- 
dening tool steel 02 or 06. 

Springs are made of carbon steels in a range from 
1065 to 1085, also occasionally up to 1095; and 
from alloy steels 5150, 6150, and 9260. Springs 
that are manually wound in the shop are made from 
spring wire; that is, high carbon steel wire common- 
ly known as music or piano wire, heat treated to 
the proper mechanical properties. 

Dowel pins are made of carbon steel, either 1035 
or a high carbon steel hardened to 60 Rockwell C, 
Keys for alignment of parts within the fixture or 
for aligning the fixture on the machine table, are 
made from 1045 carbon steel. If hardened surfaces 
are required for wear resistance, they are made from 
a tool steel. 

Assembly screws and bolts are made of various 
grades of machine steel, however, it is preferred to 
use hexagonal socket head cap screws made of heat 
treated alloy steel, usually with 160,000 to 170,000 
psi (1 100 to 1170 N/mm 2 ) tensile and 100,000 to 
105,000 psi (690 to 724 N/mm 2 ) shear strength. 
They require less space and are commercially 

Ch. 3 



The straps in clamping devices are made of 1020 
carbon steel for average conditions, and of 1035 
carbon steel in such cases where a lower strength 
material would require excessive dimensions. The 
tips of the straps are often case hardened. Hardened 
washers are used under the nuts in these devices. 
The bolts are made of 1020 carbon steel; for large 
loads they are made of a heat treated low alloy 
steel to avoid excessive dimensions. The nuts are 
made of a free-machining low carbon steel and case 
hardened for heavy loads. Fulcrum pins for swing- 
ing clamps are made of carbon steel, or tool steel, 
heat treated to 54-58 Rockwell C. Large pins are 
made of case hardened, low carbon steel. Eccentrics 
and cams are made of low carbon steel, case har- 
dened to at least 60 Rockwell C. 


Small secondary parts such as hand knobs, crank 
arms, etc., are made of malleable iron. Large cast- 
ings (and sometimes these are very large), are used 
for parallels, raiser blocks, fixture bases, and in some 
cases, for fixture bodies; particularly where vibration 
damping capacity and absence of stresses are of im- 
portance. Castings in the first category are not se- 
verely stressed and are made of gray cast iron in 
classes 20, 25 and G2000 with 20,000 to 25,000 psi 
(138 to 172 N/mm 2 ) tensile strength. Larger fix- 
ture bases, and fixture bodies in general, are made 
of gray cast iron in classes 30, 35, and G3500 with 
30,000 to 35,000 (207 to 241 N/mm 2 ) tensile 
strength. Very large fixture bodies are made of 
ductile (nodular) cast iron in class 65-45-12 with 
65,000 psi (450 N/mm 2 ) tensile strength, and type 
SP80 Meehanite Ductliron with 80,000 to 100,000 
psi (550 to 690 N/mm 2 ') tensile strength. Type 
GA50 Meehanite with 50,000 psi (345 N/mm 2 ) 
tensile strength is also used. Because of its struc- 
tural homogeneity, it retains high dimensional ac- 
curacy in service. It responds to heat treatment 
and can be hardened locally or on the surface by 
either flame or the induction process. The design 
should avoid thickness variations in adjacent sec- 
tions beyond the ratio of 3:1, otherwise the thick 
section will have a porous center. Steel castings are 

rarely used in fixtures. 

Aluminum and Magnesium 

These two light metals are widely used in the form 
of tooling plate. They are supplied with finish ma- 
chined surfaces, are weldable and easily machinable; 
for many purposes they are competitive with steel. 

Aluminum tooling plate is a cast product of a 
chemical composition equivalent to the 7000 series 
of aluminum alloys. They are furnace stress relieved 
and machined (scalped) on both sides to a thickness 
tolerance of ±0.005 inch, (0.13 mm), a flatness tol- 
erance of 0.010 inch on 8 feet (0.25 mm on 2 1/2 m), 
and 25 to 40 micro-inch (0.6 to 1.0 jum) surface 
finish. Standard thicknesses range from 1/4 inch 
to 6 inches (6 to 150 mm). Thicknesses up to 16 
inches (400 mm) are available from stock; above 16 
inches by special order. The material weighs 0.101 
pound per cubic inch (2.8 g/cm 3 ), and its tensile 
strength is 24,000 psi (165 N/mm 2 ). 

Magnesium tooling plate is a rolling mill product. 
It is made of AZ31B alloy containing A I, Zn, and 
Mn. It is thermally stress relieved at 700F (37 1C) and 
machined on both sides to a thickness tolerance of 
±0.010 inch (0.25 mm) and a typical flatness toler- 
ance of 0.010 inch in any 6 feet (0.25 mm on 
1.8 m). Standard thicknesses range from 1/4 inch 
to 6 inches (6 to 150 mm). The material weighs 
0.0642 pound per cubic inch (1.78 g per cm 3 ) and 
its tensile strength is 35,000 psi (240 N/mm 2 ). The 
compressive yield strength is only 10,000 to 12,000 
psi (69 to 83 N/mm 3 ). The machining of magnesi- 
um involves a serious fire hazard, because magnesium 
is combustible in the atmosphere, and the chips 
ignite easily and burn violently. 

Sintered Carbides 

Drill bushings for extraordinary long service life 
are usually made of sintered carbide materials. 
Two such materials are available; the first is class 
C-2 sintered tungsten carbide, a straight cobalt grade 
with 6 percent cobalt and 94 percent tungsten car- 
bide with a hardness of 92 Rockwell A. The second 
is FERRO-TIC 2 a machinable carbide which con- 
sists of titanium carbide particles bonded in a steel 
matrix of S-l tool steel. When the matrix is an- 
nealed, the material can be machined with conven- 
tional tools. After machining, the material is har- 
dened and tempered and the composite material 
shows a hardness of 70 Rockwell C. It can be re- 
annealed, remachined, and rehardened. 


Plastic tooling is made by casting and by lamin- 
ating. Plastic tooling materials include phenolics, 
polyesters, polyvinyls and epoxies. They differ 

2 Trade name, proprietary to Sintercast Division of Chio- 
malloy American Corp., West Nyack, New York. 



Ch. 3 

widely in physical properties. The physical prop- 
erties of the end product depend also on the mixing 
ratio, the curing time, and the curing temperature. 
The few values that will be quoted in the following, 
therefore, are representative only; the fixture de- 
signer is advised to obtain specific data from the 
manufacturer for any actual design application. 

The four types of plastics can, in principle, all 
be cast and used as laminating resins. The reinforc- 
ing material is usually glass fibers, applied in the 
form of cloth, mats, or rovings. 

Cast phenolics have a compressive strength of 
1 2,000 to 1 5,000 psi (83 to 1 03 N/mm 2 ); the com- 
pressive strength of cast polyesters varies from 
12,000 to 35,000 psi (83 to 241 N/mm 2 ). These 
plastics exhibit a significant linear shrinkage. Cast 
epoxy has a compressive strength of 15,000 to 
25,000 psi (103 to 172 N/mm 2 ) with an elastic 
limit of 5000 psi (34 N/mm 2 ), a modulus of elastic- 
ity (E) of 0.1 X 10 6 to 0.8 X 10 6 psi (690 to 5520 
N/mm 2 ), and a linear shrinkage of 0.001 Jo 0.004 
inch per inch (0.025 to 0.10 mm/mm), which is 
considerably less than the shrinkage of other cast- 
able plastics. The use of epoxies as a tooling ma- 
terial is growing rapidly. For this application, epox- 
ies are superior to most other plastics and they are 
now used almost exclusively for laminates. Epoxy 
laminates have an elastic limit of about 15,000 psi 
(103 N/mm 2 ), and a modulus of elasticity ranging 
from 1.5 X 10* to 3.5 X 10 6 psi (10,300 to 24,100 
N/mm 2 ). The shrinkage is negligible and they are 
dimensionally stable after curing. 

Rigid polyurethane foam is used as a core material 
and a backup material for fixtures constructed in 
the form of plates and shells. It has its optimum 
strength/weight ratio in the density range of 6 to 1 

pounds per cubic foot (96 to 160kg/m 3 ). Repre- 
sentative values of physical properties at 8 pounds 
per cubic foot (128 kg/m 3 ) are 200 psi (1.4 N/mm 2 ) 
yield strength, 250 psi (1.7 N/mm 2 ) ultimate com- 
pressive strength; B=> 10 X 10 3 psi (70 N/mm 2 ) at 
yield, and E has the near constant value of 7.5 X 
10 psi (52 N/mm 2 ) over the operating range from 
to 150 psi (0 to 1 .0 N/mm 2 ) compressive stress. 
Potting compounds used for fastening drill bush- 
ings in bored holes in plastic drill jigs are high- 
temperature-resistant epoxy resins. Epoxy resins 
with metallic fillers ("liquid steel," "liquid alumi- 
num") are used for repair of plastic tooling and for 
locators in fixtures for use at elevated temperatures 
(up to 500 F, or 260 C). 

Other Non-metallic Materials 

Large plane drill jigs for use in the aircraft industry 
where light weight is essential for easy handling, 
and high strength is not required, are made of flat 
sheets of various materials, such as plywood and 
laminated plastic sheets. A recently developed prod- 
uct is Benelex 3 , which is made of wood chips and 
consists essentially of cellulose fibers and lignite. 
U is hard, rigid, smooth, impervious to water and 
oil, and has excellent dimensional stability. It ma- 
chines like wood, the tensile strength is 7600 psi 
(52 N/mm 2 ), and the modulus of elasticity is 
1.3 X 10 6 psi (9000 N/mm 2 ). It is available in 
sheets in thicknesses from 1/4 to 2 inches (6 to 
50 mm). 

Trade name, proprietary to Masonite Corp., Oiicago, 

Locating Principles 


Locating Principles, Flat Surfaces 

A part without scribed lines and punched centers 
can only be located from its surfaces and this is done 
by providing them with the necessary number of 
restraints. To restrain the part on a surface against 
only one direction of motion, as was shown in Fig. 
3-1, is termed defining the part and implies the ad- 
dition of a subsequent clamping action to maintain 
positive contact with the restraining element. The 
part is single defined as long as it is restrained on one 
surface only, double defined when it is restrained on 
two surfaces, and fully defined when it is restrained 
on three surfaces. It is a condition here that the 
defining surfaces are not mutually parallel; generally, 
but not as an absolute condition, the three defining 
surfaces are perpendicular to each other. 

The rectangular block shown in Fig. 4-1 is a gen- 
eral example of defining and locating from flat sur- 
faces. The block is single defined as long as it only 
rests on the horizontal base of the fixture (position 
a), double defined when it is moved to full contact 
with the vertical longitudinal strip (position b), and 
fully defined when it is also moved endwise to con- 
tact with the end strip (position c). 

To define a part from two parallel and offset flat 
surfaces results in overde fining . Because of the tol- 
erances, the part cannot simultaneously be brought 
into effective contact on the two surfaces. It will 
either hang or tilt (Fig. 4-2), depending on the na- 
ture of the clamping. A similar situation exists for 
parts with two or more concentric cylindrical sur- 
faces. When the part has to be located on two offset 
surfaces, it can be done satisfactorily by locating 
them on three points. 


Fig. 4-1 . Defining and locating a part from flat surfaces. 

Fig. 4-2. Over defining a part from two parallel flat surfaces. 


A part may be located on, or restrained between, 
two or more surfaces such that motion is prevented 
in the two opposite directions on at least one line. 
The part is said to be nested, to be nesting, or to 
nest within, the restraining elements. In Fig. 4-3a, 
the part is fully defined and single nested, in b and c 
it is double nested, and in d it is fully nested. Full 
nesting requires that the fixture has a detachable 




Ch. 4 

cover to provide access to the interior of the fixture, 
and openings in the fixture walls to allow the opera- 
tions to be performed. 

Fig. 4-3. Single, double, and full nesting. 

Nesting requires that it be possible to move the 
part to a position between locating surfaces or 
points. On the other hand, it must also fit as closely 

as possible between the mating surfaces when in po- 
sition. Any clearance, no matter how small, will al- 
low motion between part and fixture that will gen- 
erate a certain small amount of displacement and 
misalignment. An interference fit would define the 
location without ambiguity, but does not readily 
permit the part to be moved into position. There- 
fore, the class of fit actually selected must be a 
clearance fit, equivalent to one of the tighter classes 
of these fits. For the sake of clarity, the clearances 
shown in the illustrations are grossly exaggerated. 
Nesting can take many forms. Nesting surfaces do 
not have to be parallel and opposite. If the fixture 
in Fig, 4-1 is rotated around one edge, it can be seen 
(compare with Fig. 4-4) that the part is actually 
double nested on two perpendicular surfaces when 
it is in position C. In addition, the diagonal plane in 
the part is now centered with respect to the fixture. 
The concept of centering is of great importance and 
will be discussed in detail in Chapter 6, in the sec- 
tion on Circular Locators, and in Chapter 9, Cen- 
tralizes. If the fixture is set on a corner with a 
corner diagonal vertical, then the part is fully nested 
on three corner surfaces, and is also centered. 

a b c 

Fig. 4-4. Modifications of the principle of nesting. 

The example in Fig. 4-1 is an illustration of the 
elimination of the six degrees of freedom by means 
of contact between large surfaces. It points back to 
the application of the same principle as is shown in 
Fig, 3-1, The base plane is equivalent to the first 
three locating points, the side strip is equivalent to 
the next two points, and the end strip is equivalent 
to the last point. This set of equivalences can be 
formulated as the "3-2-1 locating principle." The 
locating function of the side and end strips and 
points is somewhat different from the function of 
the base plane and base points. To underline this 
difference, all locating points above the base are 
termed "stops." 

Other equivalences, however, are possible. A set 
of two points is equivalent to one strip; a plane is 
equivalent to two parallel strips, or to one strip and 
a point (see Figs. 4-5, and 4-4a, b and c). A locating 
point is not a mathematical point, it is often a small 
flat surface (a pad). The locating elements should 

Ch. 4 



Fig. 4-5. Locating by means of two strips, or one strip and 
one point. 

be spaced as widely as possible. This open spacing 
provides the best obtainable stability against the 
acting loads (gravity, clamping and cutting forces), 
and minimizes any error that may be caused by a 
small misalignment or displacement of a locating 

The 3-2-1 Principle 

The 3-2-1 principle represents the minimum re- 
quirements for locating elements. The locators, to- 
gether with the clamps (represented by arrows C in 
Fig. 3-1) which hold the part in place, provide equi- 
librium of all forces, but do not necessarily also 
guarantee stability during machining. Usually, sta- 
bility is satisfactory if the three base buttons are 
widely spaced and the resultant cutting force hits 
the base plane well within the triangular area be- 
tween the buttons. If it hits outside of this area, 
then it generates a moment which tends to tilt or 
overturn the part. The pressure and frictional forces 
from the clamps may be able to counteract this mo- 
ment, but this solution is not considered good prac- 
tice, because vibrations and shocks from machining 
can cause the part to slip in the clamps. 

The 4-2-1 Principle 

By the addition of a fourth locator in the base, 
the shape of the supporting area can be changed 
from a triangle to a rectangle, as shown in Fig. 3- id, 
and provides the required stability. The principle 
may be termed the "4-2-1 locating principle." For 
rough castings, one of the four base locators may be 
adjustable. Such locators are described in Chapter 
12, Supporting Elements, 

If the locating surface is machined, all locators 
may be fixed, and this offers an advantage in an- 
other respect. When the part is properly seated on 
its four locators, it feels stable, but if a chip or some 
other foreign matter has lodged itself on a locator, 
or if the locating surface is warped, the part will 
rock. This is noticeable to the operator and serves 
as a warning that there is a defect in the set-up 
which must be corrected. 

Error Possibilities 

The use of large locating fixture surfaces is only 
feasible when the matching part surfaces are com- 
patible with respect to tolerances and geometry. 
This is not necessarily the case, even for surfaces 
already machined, because fixture surfaces are usu- 
ally finished to closer tolerances than are most pro- 
duction parts. The consequences of incompatible 
tolerances will be explained in Chapter 5 with re- 
spect to Fig. 5-2. 

The most common errors in part surface geometry 
are convex and concave curvature, twist, and angular 
errors. The effects of curvature and twist are shown 
in exaggerated form in Fig, 4-6, Convex curved and 
twisted surfaces will not accurately define the loca- 
tion since they may cause the part to rock. With 
curved surfaces and insufficient rigidity, the part 
may also be distorted (bent) when clamped in the 
fixture but after it is released from the clamp it will 
spring back and the previously flat new surface will 
now be curved. Even with distortion-free clamping, 
the curved part may still be insufficiently supported 
and may deflect under the cutting forces. 

;7l~) ) / > rT7\> > 

rrr) >///;>/, 

sTTV / > s J >T7 s 

r; ? > > / > / s > r 

rTT/ / s / ^ ■7 = T 7 - 

Fig. 4-6. The effects of locating from curved and twisted 

Angular errors on adjacent surfaces can cause va- 
rious cases of misalignment, particularly if the clamp- 



Ch. 4 




— ■ 



— ji— 


Fig, 4-7. The effects of angular errors on locating. 

ing system is incorrectly designed or operated. Some 
examples are shown in Fig. 4-7, where the large ar- 
row indicates a clamping force, and the resulting 
dimensional error in the locating is indicated by a 
double arrow. Perhaps the most dangerous case is 
that of Fig, 4-7d, because the error occurs at a place 
that is not easily observed. Nesting is no guarantee 
against the effect of angular errors, as is shown in 
the example of Fig. 4-8. The offset shown is per- 




mitted by the clearance in the nesting but it is 
caused by the angulaT error. 

Locating Principles, Cylindrical Locators 

Cylindrical surfaces will usually be located by 
nesting in or on completely or partly matching sur- 
faces. A fixture base with a side and an end strip 
can almost but not completely locate it, as shown in 
Fig, 4-9. In position a, the part stands on the base 
and three degrees of freedom have been removed. 
When moved to position c, two more (but not three) 
degrees of freedom have been removed. The cylinder 
is now nested in a V-bloek in the same way as 
shown previously in Fig. 4-4a, and it is also centered 
with respect to the V. The sixth degree of freedom, 
rotation around a vertical axis, has not yet been 

Fig. 4-8. The effects of angular errors on nesting. 

Fig. 4-9. Locating a cylinder against flat surfaces. 

The same incomplete locating can be accom- 
plished by placing the part inside a matching cy- 
lindrical holder— an outside cylindrical locator (see 
Fig. 4-10), but it still is free to rotate. Rotation can 
now be prevented and the part locked in position by 
means of a clamping device employing friction. If 
the significant part configuration consists entirely of 
cylinders and perpendicular planes, it has no pre- 
ferred diameter, atid any position is as good as any 
other position with respect to the machining opera- 
tions to be performed in the fixture. If, however, 
the part has a projecting or a receding surface, no 
matter how it is shaped, then it has one or several 
preferred diameters to which the machined surface 
must be related in the way determined in the part 
design, and such a preferred diameter, or diameters, 
must be held to a predetermined location within the 

Ch. 4 



i I 

i i 

Fig. 4-10. Incomplete locating by* means of a simple 
cylindrical locator. 

fixture. For this purpose the fixture must be pro- 
vided with one additional locating element which 
eliminates the sixth degree of freedom, rotation, by 
locating the preferred diameter(s). This can be done 
in a great variety of ways and a few representative 
examples are shown in Fig. 4-11. 

A cylindrical locator can also be applied to the 
inside of a cylindrical cavity and takes the shape of 
a mandrel, a plug, or a flange. Some examples are 
shown in Fig. 4-1 2. 

Two factors common to all rotational locators are: 
that they act on a point of a radius in the part, and 
that they restrain motion of that point in a tan- 
gential direction. These will be termed "radial lo- 
cators." Usually, and preferably, the direction of 
the actual contact pressure should be perpendicular 
to the radius at the point of contact, a condition 
which is fully satisfied in Fig. 4-12b; approximately 
satisfied in Figs. 4-1 la, b, and c, and 4-12a and c; 
but not, however, in Figs. 4-1 1 d and 4-1 2d. 

The radial locator may be a small pin, fitting in a 
hole, or it may be large and formed as another plug. 
Additionally, the cylindrical locator may be so small 
that it also takes the shape of a pin. Locating by 
such systems, illustrated in Fig. 4-13, is termed 
"dual cylinder location," and is widely used. 

Any location using cylindrical locators involves 
nesting, and therefore requires clearance which, in 
turn, affects the locating accuracy. As indicated in 
Fig. 4-14, sketch a, the position of the center of the 
part may vary as much as the clearance and may be 
offset from its nominal position as much as one-half 


i — * 

a ici 



Fig. 4-11. Complete locating of a cylinder by means of an 
outside cylindrical locator and a radial locator. 

of the clearance. The application of a clamping 
pressure (see sketch b), forces the offset to one side, 
but does not eliminate it, and the poor nesting at 
the contact point opposite the clamping pressure 
permits the part to shift slightly to one side or the 
other. If the part does not have a good locating 
base surface, but, for example, terminates in a point 
(as shown in sketch c), it is also subject to misalign- 
ment resulting in a maximum angular variation 6 of 
the axis direction determined by: 

2{D F -D P ) 360 

B = j radians ■" — -— - 

D F - D p 


once again confirming the fundamental rule that 
locating points should be as far apart as possible. It 



Ch. 4 



Fig, 4-12. Complete locating of a part fay means of an 
inside cylindrical locator and a radial locator. 

is strongly recommended, that the locating points 
be placed in mutually perpendicular planes. If a 
locating plane is inclined against the perpendicular, 
as shown in Fig. 4-15, a transverse force component 
is generated that tends to lift the part from the base 
points. A dirt accumulation on the locator of thick- 
ness T produces a locating error E=T when the lo- 
cating plane is perpendicular, but a larger error 
~ cos~a occurs wnen tne locating plane is inclined 

an angle a against the perpendicular. 

Offset and misalignments, as discussed above, are 
eliminated by the use of conical (tapered) locators, 
because they do not require clearance but provide 
positive contact. They belong to the class of cen- 
tralizing devices to be discussed in Chapter 9. 




Fig. 4-13. Examples of dual cylindrical location. 

Fig. 4-14. The effect of clearance in cylindrical locating. 

Ch. 4 



Fig. 445. The effects of locating against a perpendicular 
and an inclined plane. 

Preparation for Locating 


Locating Un machined Surfaces 

One basic purpose of a fixture is to produce parts 
that are within specified tolerances. It is the ma- 
chined surfaces on the individual parts that define 
and determine the distances to all principal axes and 
other system lines and planes within the finished 
product. It is obvious that all such dimensions must 
be correct, of course, but it is also necessary that 
any remaining unmachined surfaces maintain their 
proper location relative to system lines and to each 
other to avoid interference with each other and with 
moving parts of the- machine, to secure the required 
material thicknesses, and to provide uniform ma- 
chining allowances with full cleanup on all machined 
surfaces, A drastically exaggerated example of a 
violation of this rule is shown in Fig. 5-1 . 

Fig. 5-1, Correctly and incorrectly located center lines. 
The cylinder to the left was machined with the 
center lines correctly located with respect to the 
outer surfaces. The cylinder to the right was ma- 
chined with a gross error in the location and the 
direction of the center line with respect to the 
outer surfaces. 

In job shop production, these conditions are met 
by the layout of the parts prior to machining. Sys- 
tem lines and centers are scribed and punched into 

the surfaces of the part which is then set up in the 
machine tool by measurements taken to these lines 
and centers. One important purpose of a fixture is 
to eliminate this layout operation; the raw part usu- 
ally comes to the fixture without such lines, centers, 
or other markings, and all locating has to be done 
from the surfaces and contours as they exist. It is 
therefore important for the design of the fixture, 
and particularly for its locating elements, to know 
the dimensional tolerances that may be expected (or 
even better, may be guaranteed) on the raw part. 

They will vary from case to case, according to ap- 
plication and purpose of the product, from plant to 
plant, and from supplier to supplier. In the specific 
case, however, the applicable tolerances will nor- 
mally be made available to the fixture designer. 

Tolerances will usually be fairly consistent within 
each group of materials, depending on the type and 
class, and also the size of the part. General rules for 
tolerances and other dimensional variations are pre- 
sented in the sections following. They will be found 
useful for the fixture designer in the absence of 
specific prescribed tolerances, and may also serve as 
a base for the valuation of any given or proposed 

Machining allowances are, in a way, related to 
tolerances and must also be taken into consideration 
by the fixture designer. The maximum tolerance on 
a surface is the theoretical lower limit for the ma- 
chining allowance. Where possible, the actual ma- 
chining allowance should be obtained from the pro- 
duction planning department or from suppliers of 
raw parts. As a substitute, an estimated value may 
be used. 

For an order -of-magnitude estimate, it may be as- 
sumed that machining allowances increase with the 
overall size of the raw part. For gray iron castings 
made in green sand molds in sizes from 20 to 100 
inches (500 to 2500 mm), average machining allow- 
ances vary from 3/16 to 7/16 inch (5 to 10 mm), 


Ch. 5 



they are a little higher Cor surfaces located in the 
cope, a little lower for surfaces located in the drag. 
Practice varies between different foundries; some 
consider 1/8 inch (3 mm) as the minimum machining 
allowance, also for smaller castings. Malleable iron 
and nonferrous alloy castings require 33 percent 
less, and steel castings 50 percent more, than gray 
iron castings. 

For forgings, the weight W (pounds or kg) is the 
parameter by which the machining allowance may 
be estimated. Small hammer and press forgings 
(hand forged) require from 1/16 to 1/8 inch(l 1/2 
to 3 mm) on each surface. For this type of forging 
(from 15 pounds [7 kg] , and up) the allowance per 
surface can be taken as 

0.05 %/l7inch 

1,65 VH-" mm 

where: W = weight in pounds, and W — weight in 

For closed die forgings (drop forgings and other 
machine forged parts) the allowance required is from 
40 percent (for solid and bulky shapes) to 60 per- 
cent (for elongated shapes) of the value estimated 
for a hand forging of the same weight. Minimum 
allowance for all forgings is 1/32 inch (0.08 mm) 
because of scale pits and other localized surface de- 
fects and decarburization. 


A casting is by no means a mathematical repro- 
duction of the pattern; not even of the mold cavity. 
Some cast materials will shrink, others will expand 
during solidification. All of them shrink during the 
subsequent cooling period; the resulting total shrink- 
age depends on type and composition of the metal, 
the pouring temperature, and the cooling rate. 
Slight variations in the composition may occur from 
charge to charge and can affect the shrinkage. Un- 
even shrinkage often results from differences in wall 
thickness and may cause warping. 

Common values for shrinkage are shown in the 






Gray cast iron 

1/8 in. per foot 


Same, heavy sections 


White (chilled) cast iron 

. . , 


Malleable cast iron 


Cast steel 

1 /4 in. per foot 



3/16 in. per foot 


Material {cant.) 










small castings of 

3/32 to 5/32 in. 

simple design 

per foot 


large castings or com- 

1/12 to 1/8 in. 

plicated shapes 

per foot 

ss0.7 to 1 

Al-Si alloys 

up to 1.8 

Aluminum alloys for 

automotive pistons 

1.5 to 1.7 


3/32 to 5/32 in. 

per foot 

swO.Sto 1.3 

On large castings the apparent shrinkage will be less 
than the metallurgical shrinkage, because the pattern 
is rapped in the mold before it is drawn and thereby 
slightly expands the mold cavity. This is of signifi- 
cance for large castings only. 

With respect to warping, only a few general rules 
can be formulated. The complete process of dif- 
ferentiated shrinkage rates during solidification is 
complicated. Heavy sections, and sections that are 
shielded against loss of heat, will lag behind during 
cooling, and the end result is that such sections will 
show increased apparent shrinkage. An I-beam-type 
gray iron casting with one thick and one thin flange 
will, consequently, be concave lengthwise (hollow) 
on the side of the thick flange. An upper limit for 
the maximum deflection r raax of such a beam of 
length L and height H is: 


' ma * 3200 H 

Greater warpage may be expected for malleable and 
steel castings, A channel- or U-shaped section may 
open up at the top, because the bottom shrinks and 
the free edges are held in position by the mold or core. 

The lower limit for tolerances on castings can be 
taken as one-half of the shrinkage. This assumes 
favorable conditions, such as regular shapes without 
tendency to warping. However, while close toler- 
ances sometimes may be desired for some functional 
reason, the economical viewpoint calls for the most 
liberal tolerances that design considerations can al- 
low. Unnecessary close tolerances add to cost and 
increase the scrap hazard. Any conscientious pro- 
duction man will select the widest tolerances that he 
can get away with, and the fixture designer should 
be aware of that. 

Representative and rather realistic tolerances are: 



Ch. 5 


Iron castings, gray, white, 
and malleable: 




small castings 
large castings 

up to ±0.4 


Permanent mold castings 
on dimensions within 
one mold part: 
aluminum and 




copper and copper- 
base alloys 




Permanent mold castings 
on dimensions perpen- 
dicular to the parting 
plane or between core 
and mold 




Die castings 
on dimensions within 
aluminum and 

+ . 

15% to ±.25% 


zinc-base alloys 


1% to ±.25% 


copper and copper- 
base alloys 




lead- base alloys 
tin-base alloys 


for dimensions across 
the parting plane 



Tor dimensions deter- 
mined by a movable 






A more specific set of rules, applicable to green- 
sand iron castings up to 16 inches (400 mm) in 

size, is the following: 


Overall external dimensions 
within the same part of the 
mold parallel to the parting 

On dimensions perpendicu- 
lar to parting plane 


+0.030 inch (0.8 mm) for the 
first 3 inches (75 mm) ±0.008 
inch (0,20 mm) for each ad- 
ditional inch (25 mm) 

from +0.02 (for small cast- 
ings) to +0.06 (for large cast- 
ings) inch per inch (mm/mm) 


Cored (usually internal) 

Core location 

Concentricity of a cored 


of maximum overall casting 

+0.020 inch (0.5 mm) for the 
first 3 inches (75 mm) ±0.012 
inch (0,30 mm) for each ad- 
ditional inch (25 mm) 

±0.050 inch (1.3 mm) for the 
first 3 inches (75 mm) ±0.008 
inch (0.20 mm) for each ad- 
ditional inch (25 mm) 

0.090 inch (2.3 mm) T1R 
(total indicator reading) 

The values are for oil-sand baked cores. For shell 
cores, the accuracy is about 25 percent better. A 
comparison of these values should warn the fixture 
designer that critical conditions are more likely to 
be found on internal than on external surfaces. 

For steel sand castings, the tolerance for dimen- 
sions within the same part of the mold parallel to 
the parting plane can be taken as +0.080 inch (2.0 
mm) for the first 4 inches (100 mm) plus 0.006 inch 
(0.15 mm) for each additional inch (25 mm). The 
minus tolerance can be taken as one-half of the plus 
tolerance. The tolerances on dimensions perpen- 
dicular to the parting plane are from 100 percent 
(for small castings) to 50 percent (for large castings) 

For sand-cast aluminum, magnesium, and copper 
alloys, the general tolerance is ±0.005 inch per inch 
(mm/mm), minimum ±0.015 inch (0.38 mm). 

Casting tolerances, as listed above, do not apply to 
a dimension measured over the "gate." The gate is 
the passage leading to the mold cavity and also in- 
cludes any remnant of solidified metal from that 
passage. The gate is broken off as the casting is 
shaken from the mold, or sawed or sheared off later, 
and the remnant gate is usually cleaned up by grind- 
ing; either flush or to a tolerance that may range 
from ±0.030 inch (0.8 mm) and up to ±0.180 inch 
(4.6 mm) for large gates. 

A common feature of the configuration of pat- 
terns, and therefore also of castings, is the "draft," 
which is a slightly tapered clearance applied in vary- 
ing degrees to all surfaces that would drag against 
the mold during pattern withdrawal. The amount 
of draft is usually selected and applied by the foun- 
dry (the pattern maker), but the fixture designer 
must remember that draft is usually not shown on 
part drawings. Internal surfaces require larger draft 
than external surfaces. Larger draft is required when 
the mold is lifted from the pattern than when the 
pattern is drawn from the mold. 

Ch. 5 



Average draft values are: 
Casting Method 
I ; or pattern drawn from mold: 
external surfaces 

Amount of Draft 

100 V 2 J 

internal surfaces 

Jo ^ 

For mold lifted from pattern: 

external surfaces 


internal surfaces 


Deep castings do not permit a very large draft as it 
would too greatly distort the dimensions. 

Minimum values are: 
Casting Method 
For pattern drawn from mold: 
external surfaces, curved 

external surfaces, flat 

ribs and webs, curved 

ribs and webs, flat 

small holes 

For mold lifted from pattern: 








u " t0 20 

A rough measure of uniformity in eastings is pro- 
vided by some average weight tolerances, which, for 
gray iron castings, are 5 percent when made from 
solid patterns, 10 percent when made with sweep 
patterns, and for malleable iron castings, 5 percent 
when machine molded, and 10 percent when hand 

The uniformity and accuracy of castings (gray 
iron, malleable iron, and modular or ductile iron) is 
higher from permanent molds, and molds with me- 
tallic cores and inserts, than from sand molds; higher 
from machine molding than from hand molding; 
higher from dry-sand molds than from green-sand 
molds; and significantly higher when the castings 
are made in shell molds. 

Castings must always be expected to show various 
minor irregularities which must be tolerated and do 
not justify rejection except in extreme cases. Typi- 
cal examples are a mismatch at the parting line be- 
tween cope (upper mold) and drag (lower mold), 
flash or fins at the same parting lines and along 
edges of cored cavities, remnants of the gates, dis- 
placement of cores resulting in uneven wall thick- 
ness and machining allowance, and displacement of 
loose pattern pieces resulting in off-set bosses, ears, 
ribs, and the like. 


Handmade hammer forgings will, as a rule, not 
be manufactured in quantities that justify machining 
in a fixture. However, hammer and press forged 
parts from ferrous and nonferrous metals are used in 
moderate quantities in various industries such as the 
weapon and aerospace industries, and these forgings 
may need fixtures because of intricate and accurate 
machining requirements. The most common forged 
raw parts are impression die forgings which, again, 
may be drop forgings (closed die forgings) and up- 
set forgings. 

For estimating forging tolerances, materials can be 
classified by stiffness as follows: Low stiffness- 
aluminum, magnesium, copper, and brass; Medium 
stiffness— carbon and low alloy steel, stainless steel 
(400 series); and High stiffness— stainless steel (300 
series), titanium, super-alloys, and refractory metals 
(Columbium, Cb; Molybdenum, Mo; Tantallum, Ta; 
Tungsten, W). Tolerance data listed in the following 
without material specification may be applied to all 
three classes. 

Hammer and press forgings are seldom fully free- 
hand forged, but are made with the use of flat and 
simple open-face dies. For such forgings, tol- 
erances can be estimated from nominal dimensions 
and weight. 

For elongated shapes of length L (inches or mm) 
and any transverse dimension (width, height, diame- 
ter, etc.) D (inches or mm), estimated tolerances are: 

on length— 

T L =±[0.05 + 0,003(1+ 10/>)] inch 
T L =± [1.3 + 0.003 (L + 10 D)] mm 

on a transverse dimension- 

T D = ± [0.02 + 0.028 (D + ^ /,)] inch 
T D =+ [0.5 + 0.028 (0 + ^jL)] mm 



Ch. 5 

For a part of weight W {pounds or kg) and unspeci- 
fied shape, the estimated tolerance is: 

T = ±0.05 l/w inch or T=±\.6s\fw 


On closed die forgings the die cavity dimensions 
and the shrinkage control all dimensions inside one 
die block and parallel to the parting plane, such as 
width; length; diameters; etc. Parts formed in two 
die blocks may exhibit a mismatch which affects 
(adds to) the overall part dimensions. 

Thickness dimensions, as measured perpendicular 
to the parting plane, are likewise controlled by the 
die cavity dimensions and the shrinkage, but a more 
significant factor is the degree of die closure- which 
again depends on the amount of excess stock and 
how well this is forced out into the flash. Thickness 
dimensions may, therefore, be iess accurate than are 
other dimensions. 

Die cavity dimensions depend on the initial ac- 
curacy to which the die was sunk and polished, and 
the amount of subsequent wear. Initial dimensions 

Table 5-1a. Die Forging Tolerance Data- 

-English Units 


Stainless Steel 














Co, Mo, Ta, W 

Area in 
Parting Plane, 

Wear Factor 











Square Inches 

Thickness Tolerance, inch 









































5 16 


1000 and over 







Forging Weight 

After Trimming, 

Mismatch Tolerance, inch 


to 5 



































1000,1 and over 





Forging Weight 

After Trimming, 

Flash Extension, Max., inch 


to 10 



































1000.1 and over 





*Co = Cobalt, Mo = Molybdenum, Ta = Tantalum, and W = Tungsten 

Ch. 5 



Table 5-1 b. 

Die Forging 

Tolerance Data-SI (Metric) Units 


Stainless Steel 



Metals? 1 









Co, Mo, Ta, W 

Area in 

Parting Plane, 

Square mm 

Wear Factor 





0.008 1 0,009 





Thickness Tolerance, mm 

Oto 6,500 







6,500 to 20,000 







20,000 to 32,500 







32,500io 65,000 







65,000 to 323,000 







323,000 to 645,000 







645,000 and over 







Forging Weight 

After Trimming, 

Mismatch Tolerance, mm 

kg, approx. 

to 2.3 





2.4 to 11.3 





11.4 to 22.7 





22.8 to 45.4 





45.5 to 90.7 





90.8 to 226.8 





226.9 to 453.6 





453,7 and over 





Forging Weight 

After Trimming, 

Flash Extension, Max., mm 

kg, approx. 

to 4.5 





4.6 to 11.3 





11.4 to 22.7 





22.8 to 45.4 





45.5 to 90.7 





90.8 to 226.8 





226.9 to 453.6 





453.7 and over 





*Co = Cobalt, Mo = Molybdenum, Ta = Tantalum, and W = Tungsten 

can be held to relatively very dose tolerances which 
are considered included in the shrinkage tolerances. 
However, dies are also subject to severe wear and 
are, for economic reasons, allowed to wear consider- 
ably during their useful service life. 

Shrinkage tolerances, also known as "length-width" 
tolerances, are: ±0.003 inch per inch (mm/mm) of 
nominal dimension. Wear tolerances on external 
and internal dimensions are: wear factor (from 
Table 5-1, below) multiplied by greatest external 
dimension (length or diameter). On external dimen- 

sions, the wear tolerance is plus, on internal dimen- 
sions it is minus. Wear tolerances do not apply to 
center-to-center distances. 

Thickness tolerances are based on part area in the 
parting plane and can be taken from Table 5-1. 
Table values apply to parts not exceeding 6 inches 
(150 mm) of depth within any one die block, as 
measured perpendicular to the parting plane. For 
such parts of forgings that exceed this limit, an ad- 
ditional tolerance is applied, equal to: ±0.003 inch 
per inch (mm/mm) of any such dimension. 



Ch. 5 

Thickness tolerances are always positive, meaning 
that incomplete filling of the die cavity is not 

Mismatch tolerances and flash extension, which is 
the maximum distance that the flash may protrude 
from the forging body, are both positive; they are 
based on the forging weight after trimming, and can 
be taken from Table 5-1. Flash thickness ranges 
from 1/16 to 1/4 inch. 

Straightness tolerances mean the limitation im- 
posed on deviations of surfaces and centerlines from 
the nominal configuration and are added to pre- 
viously estimated tolerances. Forgings and parts 
within a forging can be classified by shape as elon- 
gated, flat, or bulky, and one forging may well com- 
prise parts belonging to more than one class. 

Straightness tolerances are: 
for elongated shapes— 0.003 inch per inch (mm/ 

mm) of length 
for flat shapes— 0.008 inch per inch (mm/mm) 
of length, width, or diameter. 
Bulky parts require no straightness tolerance. 

The values are for medium stiffness materials and 
assume that the forgings have been mechanically 
straightened as required. For low stiffness materials, 
deduct 33 percent; for high stiffness materials, add 
33 percent. 

All die forgings must have draft. In some extreme 
and special cases (in aluminum and magnesium for- 
ings of the extrusion type) the applied draft may be 
1 degree or even zero. However, the most common 
values on external surfaces are 5 to 7 degrees for 
medium-stiffness materials, down to 3 degrees for 
low-stiffness materials, and up to 10 degrees for 
high-stiffness materials. Internal surfaces (pockets) 
require higher drafts, from 10 to 13 degrees. All 
drafts carry a +2, —1 degree tolerance. 

Overall tolerances for closed die forgings range 
from 5 to 15 percent of thicknesses, and from 0.5 
to 1.5 percent of widths, lengths, and diameters. 
In comparison, tolerances on upset forgings are 25 
percent higher on axial lengths and flange diameters, 
but 25 percent less on some individual intermediate 
axial dimensions, such as flange thicknesses. Mis- 
match tolerances are the same. Cavities require a 
TIR concentricity tolerance of 1.3 percent of cavity 
diameter. Upset forgings do not show flash and, in 
many cases, require little or no draft. 

Tolerances quoted are "commercial." Finer tol- 
erances, known as "close" can be obtained. The 
values are approximately 33 to 50 percent less than 
"commercial." Calculated tolerances are rounded 
off to two decimal places, then converted to nearest 
higher 1/32 inch (1 mm) and entered on drawings. 

The fixture designer must be prepared to encoun- 
ter some minor defects which are considered ac- 
ceptable and passed by inspection— such as scale pits, 
shallow depressions caused by scale accumulation; 
mistrimmed edges, where the flash protrudes un- 
evenly around the forging; small fins and rags, driven 
into the metal surface; cold shuts, produced by ma- 
terial folded against itself; small unfilled areas; and 
conditioning pits, where surface defects have been 
ground away. 

The dimensionally most reliable configurations 
are those formed within one die block, and flat sur- 
faces parallel to the parting plane. All forgings pro- 
duced within one life period of the die are usually 
very uniform. The same applies to sheared flash 
contours. Slight differences may be expected when 
a die is reconditioned, and also if more than one 
die is in use. 


Production parts to be machined in a fixture will 
usually be fabricated by methods entailing closer 
control and better uniformity than job shop welded 
parts and can therefore be made to closer tolerances, 
particularly when the welding is performed in fix- 
tures. Tolerances on finished welded parts depend 
largely on the distortion during and after welding. 
The tolerances obtained must be ascertained from 
case to case, and only broad and general statements 
can be made about them. 

Automatic welding results in less distortion than 
hand welding. Arc welds distort less than gas welds. 
Heavy welds distort more than light welds, but 
heavy sections distort less than light sections. On 
the other hand, weldments from light sections are 
easier to straighten mechanically. Resistance welds 
distort less than fusion welds. Least distortion is 
found in flash- butt welding, where length tolerances 
can be held to ±0.02 inch (0,5 mm). When the dies 
are not self-centering, a maximum offset equal to 
the sum of the tolerances on the part diameters or 
thicknesses may be expected. 

In the absence of specific information, tolerances 
for resistance weldments can be taken as for die 
forgings, and tolerances for arc welded parts can be 
taken as 50 percent of the tolerances for castings of 
comparable dimensions. 

Torch-cut parts will display the thickness toler- 
ances of the stock material with an addition for the 
burr which, after proper cleaning for slag, may be 
from 0.01 to 0.06 inch (0.25 to 1.5 mm) on either 
side. Contours can be held to +0.015 inch (0.38 
mm) on small parts, and ±1/16 inch (1.5 mm) on 

Ch. 5 



large parts with automatic and tracer control, and 
±0.1 inch (2.5 mm) with manual feed. The cut 
edges may deviate 1/4 degree from the perpendicular 
position. With inert-gas tungsten cutting at high feed 
rates, edges may be beveled as much as 5 degrees. 

Mill Products 

This class comprises rolled, drawn, and extruded 
shapes. Detailed tolerances are available from sup- 
pliers' catalogs, a few illustrative examples only, are 
shown in the following: 


Steel red, low car- 
bon and low al- 
loy, round or 
Hot Rolled 
I -inch diameter 

or side 
2-inch diameter 

or side 
4-inch diameter 

or side 

Cold finished 

1-ineh diameter 

or side 
2-tnch diameter 

or side 
4 -inch diameter 

or side 

Tolerance, inch 





Round I Square 

Aluminum rod, 
round or square: 

1-inch diameter 

or side 
2-inch diameter 

or side 
4-inch diameter 

or side 

Aluminum hollow 
shapes, extruded 

wall thickness 
overall dimen- 
sions on a 
hollow section 

—0.002 -0.004 
-0.003 -0.005 
-0.005 -0.006 




-0.003 -O.005 
-O.004 -0.006 
-0.005 -0.007 

Noie: Minus tolerances only. 


Round Square 

±0.006 ±0.016 
+ 0.031 



Cold Finished 

Round Square 

±0.002 ±0,0025 
±0.004 ±0.005 


:15% of nominal 

1.5-2.5% of nominal 

With respect to tubes and pipes, the fixture de- 
signer should know that they not only have diam- 

eter tolerances, but also liberal tolerances on out- 
of-round and wall thickness variations. 

Press Products 

This class comprises sheet metal parts produced 
by shearing, punching; stamping, drawing, and press- 
ing with dies in a mechanical press. Some basic 
tolerances are usually very close. Thickness toler- 
ances for cold rolled carbon steel sheets range from 
below ±0.001 inch (0.03 mm) to ±0.005 inch (0.1 3 
mm) for thicknesses up to 1/4 inch (6 mm). Con- 
tours of punched flat parts may vary 0.001 to 0,002 
inch (0,03 to 0.05 mm) as long as the same tool is 
used without reconditioning. The same may be ex- 
pected for small stamped and drawn parts. 

Shapes formed by bending may show different 
springback. Drawn parts will have thickness varia- 
tion and, if not trimmed, a scalloped edge contour 
(earing) from planar anisotropy in the stock. All 
sheared edges have a burr on the exit side, and for 
thicknesses above 1/8 inch (3 mm), also a rounded 
entrance edge. 

Apart from these variations, parts produced in the 
same tool will come out with a high degree of 

Machined Parts 

Machined surfaces have closer tolerances than raw 
parts and are therefore, a priori, more suitable for 
locating a part within a fixture for further machin- 
ing. They should not, however, be indiscriminately 
accepted for this purpose. The basic requirement is 
that the tolerance on the already machined surface 
must be satisfactory for the correct tolerance to he 
obtained in the following operation within the fix- 
ture. As an illustration (see Fig. 5-2) assume sur- 
faces A and B are already finished to a tolerance of 
±0.005 inch (0.13 mm), and surface C must hold 
±0.002 inch (0.05 mm) against B. Surface A, pre- 
senting a wide bearing area, would appear desirable 
for locating but the presence of the ±0.005 inch 
(0.13 mm) tolerance from B to A prohibits machin- 
ing of C to ±0.002 inch (0.05 mm) from B, no mat- 
ter how close tolerance r is taken, and A must there- 
fore be rejected as the locating surface. 

Blueprint tolerances, if uncritically accepted with- 
out part inspection, could cause the fixture designer 
many disappointments. Nominally plane surfaces 
could be convex, concave, or twisted, from improper 
clamping; gradual tooi wear; inaccurate setting of a 
milling cutter; or distortion (warp) from stress re- 
lief. Broached configurations might be offset or 



Ch. 5 






y ////// 

= ±t 





a ±0.005 

Fig. 5-2. Consideration of tolerances in locating. 

tilted, due to the elasticity of the broach. Ground 
surfaces on thin parts could show heat distortion. 
Nominally square edges and corners could be out-of- 
angle from incorrect clamping. Sawed surfaces, even 
when machine sawed, are neither straight, flat, nor 
dimensionally correct. All machined edges will have 
a burr on the side of tool exit. 

Heat Treated Parts 

Such parts may distort and show relatively large 
deviations from nominal shape and dimensions. Al- 
most any geometrical element may be affected, 
Overall dimensions, including center distances, may 
increase or decrease, hole diameters may become 
larger or smaller, circles may go out-of-round, and 
flat or straight parts may curve or twist. The dis- 
tortions cannot be predicted except in rather general 
terms and may well vary from piece to piece. Con- 
trol of distortion requires careful stress relief of 
parts prior to hardening, and depends also on the 
experience of the heat treater and his skillful appli- 
cation of time-honored tricks of the trade. More re- 
liable and consistent control of heat treat distortion 
is effected by having the part clamped in a fixture 
during the process. 

As a rough estimate, tolerances required to cover 
for heat treat distortion can be taken as ±0.05 to 
±0.15 percent, additive to prior part tolerances. 

Plastic Parts, Molded 

Parts made in various sizes from plastics (thermo- 
plastic as well as thermosetting) are widely used and 
are manufactured in large quantities. They have ex- 
cellent surface quality as formed, but that is not 
necessarily identical with high dimensional accuracy, 
because of shrinkage, uniform or nonuniform, and 
sometimes dimensional changes during aging. While 
they can be formed (cast or pressed) in the mold to 
finished dimensions for many applications, other 
purposes may require some machining operations 
such as drilling and other processing of holes, where 
hole location is critical, or grinding of flat surfaces, 

and the fixture designer may well encounter the as- 
signment of designing fixtures for these materials. 
The amount of shrinkage varies with the type of 
plastic material and filler, and may in extremes, 
range between 0.001 and 0.012 inch per inch (mm/ 
mm) of nominal dimension. Representative average 
values for single-cavity hot molds, taken as the di- 
mensional difference of mold and part at ambient 
temperature, are: 


Phenolic with wood-flour filler, and urea 

Cellulose acetate 

Phenolic with fabric or asbestos filler, 

and methyl methacryiate 

inch per inch 
(mm /mm) 

0.006 to 0.010 
0.002 to 0.010 

0.002 to 0.006 
0.001 to 0.003 

Resulting tolerances can be taken as follows: 

Parallel to parting plane 
Perpendicular to parting 

Warpage, perpendicular 

to nominal surface 

±0.005 inch per inch (mm/mm) 
add 0.015 inch (0.38 mm) 
±0.003 inch per inch (mm/mm) 

The nominal dimension L (inches or mm) can also be 
taken into account in the tolerance T by the em- 
pirical formula: 

T= 0.006 \/T inches or r=0.03 Vimm 

Somewhat higher tolerances should be selected for 
center-to-center distances of bosses or molded holes, 
and for multiple cavity molds, and should be doubled 
for cold-molded parts. 

Some plastics can be molded without draft, when 
generous fillet radii are provided. Others may re- 
quire a small draft of 1 to 2 degrees on smaller 
pieces, up to a total maximum draft of 0.04 inch 
(1 mm) on the side. 

Laminated plastic parts are formed over or inside 
a die by building up consecutive layers of impreg- 
nated fibrous material (frequently glass fiber cloth) 
with a liquid resin as the impregnating and adhesive 
material. They are cured at moderate pressure that 
is provided by means of an evacuated bag. The glass 
fiber reinforcement is strong and rigid and the quan- 
tity of resin used is small, thus the dimensional tol- 
erances on the molded surface are very close. The 
man in the shop will usually say that they are zero, 
however, the designer should assume a finite, but 
small tolerance, such as ±0.003 inch per inch (mm/ 
mm) for small parts with ±0.010 inch (0.25 mm) as 

Ch. 5 



the upper limit for larger parts. It should be remem- 
bered that the material is flexible and thin parts can 
easily be elastically distorted. 

Thickness is often defined by the number of layers 
and the thickness of the stock. This may vary from 
0.003 inch to 0.050 inch (0.08 to 1.3 mm) for glass 
cloth; when a greater thickness is required a more 
loosely woven matting is used. When properly ap- 
plied, the resin fills vacancies only and theoretically 
does not contribute to thickness. The process is 
manual and may not always be closely controlled. 
Consequently, some tolerance must be allowed on 
the thickness, 1/32 inch (0.8 mm) as an upper limit 
with glass cloth and 1/8 inch (3 mm) with matting. 

Plastics, Prefabricated Shapes 

The tolerances vary widely with material, shape, 
and manufacturing method, and catalogs should be 
consulted for specific information. Representative 
values for the tolerance ranges that may be expected 
are given in the chart to the right. The first figure 
refers to the smallest, the last figure to the largest 

For pressed and rolled laminated plastic plates, 
the thickness tolerances are from 0,100 inch per 
inch (mm/mm) down to 0.030 inch per inch (mm/ 
mm). Warp and twist must not exceed from 5 per- 

Kinds of Tolerances 
and Material 

Tolerance Ranges, 

Thickness tolerances for plates: 
Plexiglas, Class A 

15, down to 6, for 1 inch 
(25 mm), and above 

Class C 

double up 

Nylon, polycarbonate, and 
styrene plates, slabs, and 

Vinyl and Teflon sheets 

2 to 12 

25 for small thicknesses, 
down to 5 

Diameter tolerances: 
extruded Nylon rod 
molded Teflon rods and 

0.3 to 0.8 
0.6 to 3 

Tolerances for tubing: 
acrylic resin, on OD 
Nylon and Teflon, on OD 
wall thicknesses 

0.9, down to 0.5 
5, down to 2.5 
15, down to 5 

cent down to 1/4 percent of the plate dimension 
(length, width, or diagonal). For tubes and rods, 
the tolerances on the OD are from 1 percent down 
to 0.3 percent, and on tube wall thickness from 20 
percent down to 4 percent. The larger tolerances 
are for the smaller dimensions. 


Design of Locating Components 

General Requirements 

Locators and stops present a number of require- 
ments other than merely the proper locating of the 
part. The most important of these are: (a) resist- 
ance to wear, (b) provision for replacement, (c) 
visibility, (d) accessibility for cleaning, and (e) pro- 
tection against chips. All of these points must be 
taken into account in the selection and design of the 
locating elements. At this stage of the design de- 
velopment, the fixture designer is also advised to 
think ahead and review the aspects of the loading 
and unloading of the fixture, as previously discussed. 


The simplest method of locating, not previously 
discussed, is locating by sighting to locating lines or 
other markings in the jig. A normal prerequisite for 
the use of this method is that the part has an accept- 
able base surface on which it can rest in a stable po- 
sition in the jig. Once this is accomplished, the part 
is moved until its contour coincides sufficiently close 
with the markings and is then clamped in position. 
The method can be used for raw parts such as cast- 
ings, welded parts, and forgings, where no great ac- 
curacy is required between the part contour and the 
surfaces to be machined. Such parts involve large 
tolerances on the part contour, and for this reason, 
each marking is made with multiple lines to make 
sure that the markings are not totally obscured. 
With this simple device, it is always possible to lo- 
cate and center the part fairly well. Two simple 
examples with two different styles of marking lines 
are shown in Fig. 6- la and b. For correct locating, 
the method shown in these two diagrams depends 
entirely on the attention and manual skill of the 
operator. However, less manual skill is required in 
the modifications c and d, when the part is located 

Fig. 6-1. Locating by sighting to lines. 

by manipulation of finger screws. This dependence 
on human judgment is not necessarily always a lia- 
bility, since it also permits the operator to adjust to 
the correct location despite bumps or other local 
irregularities on the part contour. 

An example of a different technique, still based 
on sighting, is a drill jig, shown in Fig. 6-2. The drill 
plate carries sighting apertures with beveled edges, 
and the part contour is lined up with the edges of 


Ch. 6 



Fig. 6-2. Locating by sighting to edges. 

these apertures which may be round or elongated 
holes or slots. The part is here adjusted to its final 
position by means of cams and screws. 


The next logical step, again applicable to flat parts 
or parts with at least one flat or fairly flat surface, is 
to nest it along its contour or along the contour on 
its extreme ends. An example is shown in Fig. 6-3a. 
The semicircular notches provide space for the oper- 
ator's fingers for inserting and removing the part. 
The groove at the contour allows for the burr. 

The minimum clearance between nest and part is 
determined by the part tolerance and, obviously, 
permits some displacement. The nesting of irregular 
shapes is, therefore, limited to parts that are already 
manufactured with rather close contour tolerances. 
It is usually very suitable for parts punched from 
sheet and plate yet less suitable for forgings, partly 
because of the draft, partly because the contour 
where the flash has been trimmed off may be offset 
with respect to the forging body. 

The contour can also be simulated by blocks with 
V-notches (Fig. 6-3b). These are cheaper to make 
and can also be made adjustable to accommodate 
for variations in the part contour from wear or re- 
conditioning of the tool with which the contour 
was made. 

A simpler and cheaper method is nesting with pins 
(Fig. 6-3 c). All pins are shown as cylindrical. As will 
be explained later, locating pins are, in most other 
cases, provided with flat contact surfaces. This is 
frequently omitted in contour nesting fixtures when 


Fig. 6-3. Examples of nesting along a contour. 

the pins are not exposed to any substantial load, and 
also when the pin has to contact a curve on its con- 
cave side. 

Dust and chip fragments which, when accumu- 
lated, prevent proper seating and cause misalign- 
ment of the part are difficult to clean out of nesting 
fixtures, particularly the full nest type. Dirt space 
allowance is therefore required. A burr groove pro- 
vides an excellent dirt space. V-blocks and pins can 
also be undercut for the same purpose. 

Three-dimensional Nesting 

Nesting fixtures for parts with an irregular surface 
in three dimensions can be made by machining, 
which is highly expensive; and by casting, which is 
the more commonly used method. The fixture is a 
box of ample dimensions to contain the part and 



Ch. 6 

the nest is formed by sealing the part against the box 
and pouring a castable material onto the part, An 
example of this is shown in Fig. 6-4. 

Fig. 6-4. Three-dimensional nesting of an irregular surface. 

Castable materials used in nesting are plastics and 
soft metals. The plastics are phenolic tooling resin 
and epoxy, reinforced, when needed, with glass 
cloth. They are light, inexpensive, and easy to re- 
pair if required. The curing temperature is 300 to 
350F (149 to 177C). The metals used are Kirksite®, 
a group of zinc base alloys with a melting range of 
717 to 745F (381 to 396C), and poured at 850F 
(454C); various lead-lin-antimony alloys with a melt- 
ing range from 460 to 500F (238 to 260C); and 
Cerrobend®, an alloy containing bismuth and melt- 
ing at 158F (70C), that is, below the boiling point 
of water. The cast surface is ground, if necessary, 
and polished to provide some clearance. 

This type of nesting fixture is suitable for parts 
with fairly close tolerances, such as die castings and 
stampings. However, the nesting surface can be sub- 
divided by machining grooves and recesses, and re- 
duced to locating pads, as indicated by the dotted 
lines. In this way the fixture can be made to accom- 
modate parts with wider tolerances, such as ordinary 
castings and forgings. 

Integra! Locators 

For parts of simple geometry and with flat ma- 
chined surfaces of sufficiently close tolerances with 
respect to flatness and dimensions, the simplest lo- 
cating solution is to provide mating locating surfaces 
integral with the fixture. The principle, as applied 
to a cast fixture, is illustrated in Fig. 6-5. The ma- 


chined locating surfaces are indicated by /. The dia- 
gram shows large continuous surfaces as well as in- 
dividual pads. Some aspects of the use of large lo- 
cating surfaces have already been discussed. Large 
bearing areas provide excellent support for the part 
and permit a great deal of freedom in the placement 
of clamping forces without danger of elastic distor- 
tion (deflection, springing) of the part; also, as the 
bearing pressures are low the rate of wear is reduced. 
On the other hand, large locating areas require a high 
degree of accuracy in the part as well as in the fix- 
ture, for accuracy is lost if the fixture distorts as a 
result of poor stress relief. Dirt space, however, is 
only available along the perimeter, thus large sur- 
faces are apt to accumulate dirt and chip fragments. 
It is possible to subdivide large locating surfaces 
without loss of their advantages. As shown in Fig. 
6-6, the first step is to provide grooves for the ac- 
cumulation of dirt; (left) two sets of crossing grooves 
change the original surface into smaller pads without 
serious sacrifice of supporting and bearing areas; the 
individual surface areas are reduced, which also fa- 
cilitates cleaning. The next step (right), is to reduce 
the original surface to strips, and finally, not shown, 
to reduce each strip to small pads. All these changes 
facilitate the drainage of coolant as well as the ma- 
chining of the fixture; particularly the finish grinding. 

Fig. 6-5. Integral locating pads. 

Courtesy of Technological Institute, Copenhagen 
Fig. 6-6. The reduction of large locating surfaces by means 
of grooves. 

The two patterns shown are only modifications of 
details and the geometrical concept of the locating 
surface as being on one plane remains unchanged. 
There is no objection here to the use of four corner 
pads with the inherent advantage of maximum sta- 
bility, and there is, therefore, no obligation for re- 
ducing the locating surface to three points. 

Locating strips and pads are easily formed in a 
cast fixture body because they are molded by means 
of the patterns and cores. They are just as easily 

Ch. 6 



provided in welded fixture bodies. Typical examples 
are shown in Fig. 6-7a and b. 

Fig. 6-7. Examples of welded jigs with welded locating 
strips and pads (B in part b). 

Cast or welded integral locating surfaces suffer 
from a common drawback-they are not directly 
replaceable when worn. It is not practical to harden 
a cast-iron fixture (steel castings are very seldom 
used for fixtures) and the only means available for 
controlling the wear rate is to be as generous as pos- 
sible with the dimensions of the locating surfaces to 
keep the bearing pressure low. The same applies, in 
general, to welded fixtures. However, although not 
widely used, it is possible to make the locating pads 
and strips from low-grade tool steel and weld them 
into the fixture body. Apart from this, worn sur- 
faces, both on cast and on welded fixtures, can be 
reconditioned by weld-depositing a layer of material 
and remachining it to the original dimensions. The 
welding involves some risk of distortion, and a care- 
ful inspection of the fixture is required after the re- 
pair. A safer, but more expensive method, is to re- 
move the worn pads by machining and install new 
pieces made from hardened steel, secured by means 
of screws and dowel pins. 

Separate Locators 

For the reasons explained above, it is preferred to 
use separate components for locating purposes, to 
install them in such a way that they can be removed 
and replaced when worn, to provide them with a 
hard working surface, and to protect them against 
chip and dirt accumulation. 

Locators have been made from bronze, presum- 
ably because of its use as a bearing material. Loca- 
ting wear strips of synthetic sapphire have shown a 
wear resistance several thousand times that of steel. 
However, these material selections are exotic and 
highly unusual. The widely accepted rule is to make 
locators from steel, occasionally chromium plated, 
or from cast or sintered carbide. Small locators are 
made from low alloy steel, heat treated to 41-45 
Rockwell C, large locators from low carbon steel, 
Carburized and case hardened. 

Wear on Locators 

Wear is a complicated process and has been exten- 
sively studied. Most wear research is for the purpose 
of better bearing design, but the conditions in a bear- 
ing (lubrication, regular motion, cleanliness) do not 
apply to fixture locators. With dust, chip fragments, 
rust, and scale always involved in their use, the con- 
ditions on locator surfaces are far from ideal, and 
the type of wear to be expected is an intermediate 
between contact wear (metallic contact between 
clean or corroded surfaces, no lubricant, no signifi- 
cant amount of foreign particles) and abrasive wear. 
Locator surfaces do have one advantage with respect 
to wear, namely, that they are not exposed to very 
much sliding motion by the part. Motion takes 
place only during loading, and the maximum load on 
the locators during this period is only the weight of 
the part. With correctly designed clamps, there 
should be no motion when the clamping pressure is 
applied nor when the working load from the cutting 
operation is applied. 

It would be desirable if quantitative data for per- 
missible locator loads could be quoted but in general, 
they cannot. The only somewhat relevant figure is a 
value for hardened steel of 25 pounds per square 
inch (0.17 N/mm 2 ), found by French and Hersch- 
man 1 as a boundary between a lower pressure region 
with slow wear and a higher pressure region of more 
rapid, increasing wear. The curve from which this 
value is extracted is shown in Fig. 6-8. 

1 H. J . French and H. K. Herschman, "Wear of Steel with 
Particular Reference to Plug Gages," ASTM Proceedings, 
vol. 10, 1926. 



Ch. 6 





a a. 


3 2 




0.4 — 







JO 10 30 W SO 

Courtesy ofH. J. French and H. K. fferschman 
Fig, 6-8. The rate-of-weai of hardened steeL 

Example -The largest size of rest button taken from 
manufacturer's standards has a 1 1 /4-inch (32-mm) 
total diameter. Deducting for the chamfer, the ef- 
fective diameter is: 

0.92 X 1.25 = 1.15 inches (29.2 mm) 

and with three buttons, conforming to the 3:2:1 
principle, the maximum load carried within the 25 
psi pressure limit is: 

■4 X 1.1 5 2 X 3 X 25 = 78.9, or approximately 80 

pounds (36 kg) 

A very large number of parts weigh less than 80 
pounds and with the use of conventional buttons, a 
long locator service life can be expected. The fix- 
ture designer should not despair if the locator pres- 
sure significantly exceeds the limit quoted, but he 
must make ample provision for replacement of worn 
locators. No fixture is really expected to last for- 
ever, and larger parts usually do not occur in such 
quantities that locator wear becomes a great prob- 
lem. When necessary, larger locators can be de- 
signed, but under no circumstances should the de- 
signer feel obligated to employ locators with ex- 
cessive bearing areas, chiefly because it is difficult 
to keep them free of chip fragments. 

In difficult cases selecting a more wear-resistant 
material is justified. The ratio of wear resistance of 
the four materials— case-hardened carbon steel, har- 
dened tool steel, cast tungsten carbide (Stellite type), 
and sintered tungsten carbide— is 1:2:3:40. Any 
discussion of high wear-resistance applies only to 

fixtures for large quantity production. By rule-of- 
thumb it is accepted that unhardened locators are 
sufficient for tooling for 1 00 parts or less. 


The three most common types of locating "points" 
are buttons, pins, and pads. Conical points are ideal 
from the mathematical viewpoint only, and should 
not be used because they lack sufficient bearing sur- 
face area and would rapidly wear down. 

Buttons are round and have either a flat head or a. 
crowned (spherical) head, as shown in Fig. 6-9. They 
are made of steel; usually medium alloy steel or low 
grade tool steel, heat treated to 40-45 Rockwell C, 
or (larger sizes only) low carbon steel, such as AISI 
1113, carburized and case-hardened to 53-57 Rock- 
well C, the choice determined by heat treatment 
considerations. Buttons are precision parts and are, 
therefore, ground after heat treating; sufficient re- 
lief for grinding must be provided between the shank 
and the head. Flat buttons are used against ma- 
chined surfaces only; crowned buttons are primarily 
for use against unma chined surfaces, but can also be 
used for locating machined surfaces. However, they 
do not provide a well-defined bearing area. 

Buttons of these types when used as base locators 
are commercially termed "rest" buttons; when used 
for side and end stops they are then termed "stop" 

Installation of the button in the fixture body is 
done with a press fit in a cylindrical bore (reamed, 
precision bored, or ground). For this purpose the 
shank ends with a 30-degree chamfer. The bore goes 
through the fixture wall; a blind hole will trap air 
during pressing and does not permit easy removal of 
the button for replacement. The fixture surface is 
then machined to provide positive support and ad- 
ditional alignment for the head. By providing a boss 
around the hole, the machining is reduced to a spot 
facing; on a flat surface, it can be done by counter- 

While the shanks on commercial buttons are sup- 
plied with standardized tolerances, resulting in an 
oversize ranging from max, 0.0010 to max. 0.0015 
inch (0.03 to 0.04 mm) within the available diameter 
range, there is no formal standard for the interfer- 
ence required relative to the hole, nor to the hole- 
diameter tolerances. However, it is generally as- 
sumed that the hole is finished with a reamer with 
max. oversize of 0.0002 inch (.005 mm) when new. 
An analysis of these figures indicates that the fit 
actually obtained will fall in the range from inter- 
ference-fit class LN 3 to force-fit class FN 2. The 

Ch. 6 



Fig. 6-9. Locating buttons. 

class FN 2 fit represents the upper limit, which is in 
good agreement with the fact that it (the FN 2 fit) 
is about the tightest fit to be used in cast iron. It 
should be remembered that the reamer, even if it 
holds the 0.0002-inch oversize, may well produce a 
larger hole, and consequently a lighter fit, if it is al- 
lowed to wobble during the reaming operation. 2 

When a plane (or a line) is defined by three (or 
two) buttons, they are surface ground across their 
faces after installation to ensure that the plane (or 
line) is parallel to the corresponding outer surface 
of the fixture, 

With a good press fit and a machined surface on 
the fixture wall, the installation of the button is ac- 
curate, safe, and economical. It is also proposed (in 
the literature and in catalogs of fixture components) 
to use a threaded shank in a tapped hole. In this 
case, the button also has a hexagonal section for a 
wrench, as shown in Fig. 6-1Q. 3 !n general, this 
practice is not recommended. A screw thread re- 
quires clearance and is less accurate with respect to 
location and direction. The button is not locked 
and may be loosened by vibration. A fatigue failure 
or accidental overload (a blow) may break off the 
head and make the shank difficult to remove. These 

^Detailed information on definitions and classification of 
the standardized fits and numerical data for their clearances, 
interferences, and tolerances is found in Erik Oberg and 
F. D. Jones, Machinery 's Handbook (New York: Indus- 
trial Press Inc., 1971.) 19th ed., pp. 1518-1529, followed 
on pp. 1529-1538 by the metric (ISO) limits and fits. 

threaded buttons are for permanent installation and 
must be screwed in tightly; they are not intended to 
be adjustable in height. Actual adjustable stops and 
supports will be described later. 

y~ \ 



L _ 

. J 











Courtesy of E. Thaulow 
Fig. 6-10 (Left). Locating button with screw thread and a 
hexagonal section. 
Fig. 6-1 1 (Right). A hollow locating button. 

Hollow buttons (Fig. 6-11) fastened by separate 
screws, are used occasionally as they are a little 
cheaper to install. The screw head must be counter- 
sunk safely below the face of the button, which 
leaves a small cavity for the collection of chip frag- 
ments and is difficult to clean. 

Rest and stop buttons are commercially available 
in standardized dimensions. Few cases are encoun- 
tered within the range of standardized dimensions 

E. Thaulow, Maskinarbejde 
Gad's Forlag, 1930) vol. II. 

(Copenhagen: G.E.C. 



Ch. 6 

where standard buttons cannot be used; in such 
cases, and when larger sizes are required, well-bal- 
anced dimensions for stable buttons can be taken 
from the formulas below. The symbols refer to Fig. 
6-9, The principal dimension is the overall diameter 
D, Each diameter £> permits a range of heights H. 
The lower limit of the range is for the purpose of 
safely clearing the fixture base and any accumulation 
of dirt and chips; the upper limit is determined by a 
stability consideration. For equal shank dimension 
B, the crowned button has a smaller overall diameter 
£>, as no nominally defined bearing area is required. 

For flat buttons, H can be selected: 

from 1/3 D (but not less than 3/16 inch [5 mm]) 
to 4/3 D (but not more than 1 inch [25 mm]) 

B= 3/4 (D-l/8) 
L = 1/2 (£>+#) 

The formulas, except the one for B, are valid in 
English and in metric units. With metric units, use: 

B = 3/4(D-3) 

For crowned buttons, H can be selected: 

from 1/3 DtoD, and 

R = 3/2 Z) 
fl = 3/4 ZJ 


A pin is a cylindrical component that is contacted 
on its side. It follows from this function that the 
height of a pin is not a critical dimension. Buttons 
can be substituted for pins, but pins cannot be sub- 
stituted for buttons. Pins used as locators are in- 
stalled by a press fit in the same manner as a button 
with or without a shank of a reduced diameter. Pins 
are used to make a nest and, generally, as side stops 
and for locating in holes, an application which will 
be further discussed later. 

A number of typical applications of pins and but- 
tons for side stops are shown in Fig. 6-12. In most 
of the sketches, no provision is shown for dirt and 
chip relief spaces. 

Round pins (and buttons) for side stops can be 
used on concave and unmachined surfaces. For use 
on plane machined surfaces, the pin or button has a 
flat to mate the surface on the part. For high pre- 
cision, these flats are ground after installation of the 
pins in the fixture. Generally, the use of a pin as a 




Fig. 6-1 2. Typical examples of pin and button side locators. 

a. A simple locating pin used as a side stop. 

b. Same, with relieved bearing area in fixture 
base, c, The conventional use of a side stop 
button, d. A button used as a pin for a side 
stop, e and f. Pin and button with a flat locating 
surface used as a side stop. 

side stop is a little primitive. The more usual method 
of making a side stop is to use a button mounted in 
the side wall of the fixture, with its face mating the 
side surface of the part as shown in Fig. 6- 12c. Pins 
as side stops should be used only on shallow parts 
with light side loads to avoid loading the pin with a 
large bending moment. 


Pads are usually flat components made from simi- 
lar steels and heat treated to similar hardness levels 
as buttons. They are ground flat and parallel, some- 
times also ground on parts of their perimeter and 
are installed on machined surfaces in the fixture 
body. They are used primarily as base locators in 
cases where rest buttons do not provide sufficient 
bearing area, as side and end locators, and as nest 

Ch. 6 



The edges and corners of a pad are usually not 
rounded, beveled, or chamfered as are the edges on 
a button, but are only slightly broken and lightly 
polished to remove burrs and make them smooth to 
the touch. The reason for this difference is some- 
what obscure. This is a case where a design detail is 
based on habit rather than on calculation or rational 
logic. Pads located down in the interior of a cast or 

welded fixture are not chamfered on their edges as 
they are not easily accessible to the machine tool. 
Loose pads should look like fixed pads and, there- 
fore, they are also left with their corners and edges 
intact. In all fairness, it should be noted first, that 
sharp edges on a locating pad may be useful in scrap- 
ing dirt off the mating part, and second, that the 
chamfering of a pad, particularly one of an irregular 

Fig. 6-13. Fastening methods for pads and other locators. 



eft. 6 

outline, is quite an expensive operation because it 
requires considerable handwork, while the chamfer- 
ing of a button is a rapid and inexpensive screw ma- 
chine operation. 

Pads are fastened by means of screws with well- 
countersunk heads, and their position is secured by 
means of dowel pins (also other means, as required), 
since screws are fasteners only and are not capable 
of precision locating anything. The correct use of 
dowel pins follows certain rules which apply not 
only to locating pads but to any loose part to be 
permanently installed with significant precision. A 
number of representative cases are shown in Fig. 
6-13. (Lower case letters refer to the particular 

In principle, two dowel pins are required for lo- 
cating a component and they are placed as far apart 
as possible (a, c). The holes are drilled undersize 
and reamed to size after the pieces are fixed in po- 
sition by means of the screws. If the one part, such 
as a locator pad, is hardened, the holes in that part 
are reamed to size before heat treatment, and the 
mating holes in the fixture wall are reamed to the 
correct size and location through the hardened holes. 
This maybe considered a less-than-ideal compromise 
solution. A better solution, and one that is used oc- 
casionally, is to leave one section of the part un- 
hardened and to place the dowel pins in that sec- 
tion (b). 

In many cases, it is possible to reduce the number 
of dowel pins to one, namely, when other locating 
surfaces of sufficient precision are available to assist 
in defining the position of the part. A key and key- 
seat (d) or a recess (e) may serve this purpose. One 
screw, a keyseat or recess, and one dowel pin define 
a position (f). If the position in the direction of the 
keyseat or recess is not critical, a dowel pin is not 
even needed. Two screws and a keyseat will define 
the part (g). Two dowel pins substitute for the key- 
seat, assisted by four or two screws (h). Two screws 
and one dowel pin may occasionally suffice (i), 
namely, if the orientation is not critical. On the 
other hand, a part nested in a well-fitting recess, 
held with three screws and secured by one dowel 
pin (j) has an extremely well-defined and secured 
position. Parts with cylindrical shanks fitting closely 
in holes are completely defined by one dowel pin 
only (k, 1). 

As with the shanks for the buttons, dowel-pin 
holes are drilled through so that the pins can be 
driven out again, when necessary. The recom- 
mended bearing length of a dowel pin in each part 
is 1 1/2 to 2 times the diameter of the pin. 

Fig. 6-14 {Left). Standard dowel pins. 

Courtesy of E. Thaulaw 
Fig. 6-15 {Right). A tapered dowel pin with extractor 
screw thread. 

Dowel pins are cylindrical (straight), or tapered 
(Fig. 6-14). The standard taper is 1/4 inch per foot 
(1:48), ("Taper" is diameter difference divided by 
length.) Straight and tapered pins are commercially 
available. The straight type is available unhardened 
as well as hardened and ground, For permanent as- 
sembly (apart from the possibility of infrequent re- 
placement of a component) the fit of the dowel pin 
can be a press fit in each part. This serves the pur- 
pose of most fixture applications; in cases where oc- 
casional disassembly is anticipated, it is common 
practice to give the dowel pin a press fit in one part 
and a tight sliding (slip) fit in the other part. Again, 
this is not very common in fixture design practice. 
Tapered pins are easily loosened by the application 
of light pressure or a blow on the small end. They 
are, therefore, often preferred for parts that require 
frequent disassembly. However, the tapered pin 
does not produce and maintain as accurate an align- 
ment between parts as does the straight part with a 
press and a sliding fit. In extreme cases where parts 
must be disassembled very frequently, the sliding fit 
will wear in time and accurate alignment is lost. In 
such cases, a hardened and ground tapered pin gives 
much better service. A more sophisticated version 
of the tapered pin, which greatly facilitates its re- 
moval, is made with a threaded end and a nut as 
shown in Fig, 6-1 5. 4 The nut is backed off when 
the pin is driven into place. When the nut is tight- 
ened, it gently loosens the pin. A compromise pin 
design is the tapered pin with a short hexagonal 
head. When flat pads used as base locators are 
fastened by the means described, the countersunk 
screw heads offer places for the accumulation of 
chip fragments that are difficult to clean away. 
Unbroken pad surfaces can be obtained if the pads 

E. Thaulow, Maskmarbejde (Copenhagen: G.E.C. Gad's 
ForLag, .1930) vol. II. 

Ch, 6 



are fastened and located by means of screws and 
dowel pins from the reverse side and with blind 
holes. The method is somewhat cumbersome and is 
not widely used, but it is a legitimate possibility and 
undeniably it does serve the purpose of providing 
an unbroken bearing surface. 

Dowel pins are used extensively in the construc- 
tion of built-up fixture bodies, and a detailed de- 
scription of dowel pin techniques is presented in 
Chapter 15, Design of Fixture Bodies. 

Circular Locators 

Circular locators take the form of pins, mandrels, 
plugs, and recesses for inside locating; and hollow 
cylinders, rings, and recesses for outside locating. 
In principle, they are nesting devices, and as such, 
they share the two problems of jamming and clear- 
ance versus locating accuracy. 

Jamming is mainly a result of friction. If there 
were no friction, the part would always slide smooth- 
ly into the locator. The jamming process is also 
affected by the amount of clearance, the length of 
engagement, and the steadiness of the hand of the 

When jamming occurs, it always begins when the 
part has entered a short distance into an outside 
locator or around an inside locator. A case of jam- 
ming is shown in Fig. 6-16. The outer cylinder (the 
locator) has diameter W, and the inner cylinder (the 
part) has diameter W - C, where C is the clearance. 
The part has entered the locator over a short length 
L, the length of engagement. If the part is slightly 
tilted, as shown, then one side of the leading edge 
comes into contact with the inside of the locator 
and is caught by the friction. If additional pressure 
is applied to the part, it serves only to increase the 
friction and the tilt, and thus jams the part. 

ment L, there exist two critical values L x and fc Sl 
which can be calculated. Below L x and above /. 2 
there is no jamming; the range from L x to L 2 is a 
no-man's-land where jamming is possible and likely 
to occur. This area can, however, be completely 
eliminated, and the locator made jam-free, by pro- 
viding a relief groove on the locator over a length of 
at least from /,i to L 2 , Dimensions can be cal- 
culated from the general theory. The most impor- 
tant dimension islj which is determined by 

L 2 =M* 

where £t is the coefficient of friction and W is the 
width of the opening. 

The dimensions of the relief grooves can be stand- 
ardized. No such standard exists as yet in the United 
States. A German standard (DIN-Norm 6338 in 
Vorbereitung) for locating pins has been proposed 
with dimensions closely approximating those which 
can be derived from the theory and with a chamfer 
for pre-positioning. Converted to easy formulas, the 
recommended dimensions (see Fig. 6-17) are: 

Li =0,02D 
L 2 =G.12D 

1 3 ss 1/3 \/5 ( with L 3 and D in inches) 
L 3 « 1.7 y/D (with 1 3 andZ) in mm) 
d = 0.97 D 

Fig. 6-16, Jamming. 

According to general theory, the risk of jamming 
is associated with the length of engagement between 
the part and the locator. For the length of engage- 





Fig. 6-17. The significant dimension of a jam-free 
circular locator. 

The mode of action of a circular locator is modi- 
fled when it is combined with a flat locating surface, 
a plane perpendicular to its axis. The flat surface 
aligns the part and defines the direction of its axis 
and the circular locator needs only to define the 
location of the axis with the result that its length 
can be reduced. It is always safe to make the total 
length less than the previously defined length L t , 
but it is by no means necessary because the outer 



Ch. 6 

dimension of the part is also a factor in determining 
the maximum possible angle of tilt, as seen in Fig. 
6-18, A point A on the locating surface of the part 
can swing in a circle around a center B on the outer 
perimeter of the part. Any length A y D of the loca- 
tor that makes it stay within the circle around B is 
jam-free, even if it is greater than L lt and the di- 
agonal CD is longer than the diameter AC inside 
the part. 

Fig. 6-18. The geometry of a jam-free circular locator in 
combination with a flat locating surface. 

Any circular locator of a shape contained inside the 
sphere will locate jam -free. Such a locator, consist- 
ing of two opposed conical surfaces joined by a nar- 
row cylindrical band, is shown in Fig. 6-1 9b. This 
is a solution with practical applications, 

An entirely different type of modification of a 
cylindrical locator is shown in Fig. 6-20. Three flats 
are machined on the cylinder, leaving three circular 
lands 120 degrees apart. To provide sufficient bear- 
ing area, the width of each land is taken as 30 de- 
grees. This cut cylinder is now used as an internal 
locator and mated with an external part which is as- 
sumed to be longer than the locator. With the same 
letter symbols as in Fig. 6-16, the outer cylinder, 
shown at the left, has diameter W, and the inner 
circle through the three lands (shown at the right) 
has diameter W - C, where C is the diametral clear- 
ance. In the concentric position there is a radial 

clearance of — on each land. This is also the distance 

A A i and, therefore, the vertical clearance at A. The 

When the circular locator is combined with a flat 
for alignment, it does not even have to be cylindrical. 
If it is made spherical (see Fig. 6- 19a) it still centers 
the part. A sphere has one and only one diameter 
and no "diagonals," and is jam-free at all angles. 
It is expensive to machine with good accuracy, and 
the spherical locator is therefore not a very practical 
solution, but it points the way to other solutions. 

a b 

Fig. 6-19. Jam-free noncylindrical circular locators. 

0.8536 CW-C) 

Fig. 6-20. A cylindrical locator with triangular relief to minimize jamming. 

Ch. 6 



radii to E and F are drawn at 45 degrees with the 
horizontal. With the parts still in the concentric po- 

sition, the vertical clearance at E and F is 1.4142 2 , 

making the total effective clearance for vertical 

f + 1.4142^ = 1.2071 C 

c , . 

To jam, first move the part a distance — up until A i 

falls on A 7 , and there is contact with the locator 
along the generatrix through A . The outer circle, 
which is the contour of the bore in the part, is pro- 
jected as the circle through A^E^Fj. Then tilt the 
part around a horizontal axis through A?, perpen- 
dicular to the axis of the locator, and located at the 
forward end of the part, with the rear end of the 
part moving down. Continue tilting until points E 2 
and F 2 , located further back in the bore, come in 
contact with points on the rear end of the locator, 
projected in points E and F. This is the position 


Fig. 6-21. Overdefining and correct defining of a part with 
more than one significant diameter. 

Fig. 6-21 Facilitating the entrance of a part with two sig- 
nificant diameters. 

where jamming may begin. In this position, the old 
dimension W is replaced by 0.8536 W (see left part 
of the figure) so that the critical length £ 2 is now: 

L 2 = 0.8536 flW 

The triangular shape has reduced the critical length 
for jamming by approximately 15 percent but has, 
at the same time, increased the effective clearance 
by approximately 20 percent and reduced the loca- 
ting accuracy of the locator by the same amount. 

Locators for parts with more than one significant 
diameter must not overdefine the part. An exag- 
gerated bad example is shown in Fig. 6-2 la. It is 
four times overdefined. The design of the locator 
can be improved in many different ways; two cor- 
rect designs are shown in diagrams b and a Loca- 
tors with more than one significant diameter must 
also be so designed that only one diameter locates 
at a time. The locator shown in Fig. 6-22a is wrong 
in that two diameters are required to catch simul- 
taneously. By increasing the length of that part 
which has the smallest diameter, the small diameter 
will enter first and help in guiding the large diam- 
eter, as shown in Fig. 6-2 2b. 



Ch. 6 

Radial Locators 

Radial locators are those that act on a radius in 
the part to prevent rotation around a fixed center. 
Instances where a "radius" is a physical feature of 
the workpiece have been discussed previously. There 
are many cases, however, where the configuration of 
the workpiece does not provide any opportunity for 
radial locating, and other means must be found for 
this purpose. Such means fall into three categories: 
keys and keyseats, dual cylinder locating, and index- 
ing fixtures. Any radial locator has a certain toler- 
ance and therefore involves the possibility of an 
angular error. With tolerance T and radius R {see 
Fig. 6-23) the angular error is: 


8 = ir radians 

Since T may be a constant, or at least a quantity 
with a fixed lower limit, it follows that radial loca- 
tors should be placed on the largest possible radii 
for the best angular accuracy. 

t-'ig. 6-23. The radius sensitivity of a radial locator. 

Keys and Keyseats 

The key with its keyseat is one of the most com- 
mon elements used in machine design for the express 
purpose of permanently locating one machine part 
radially with respect to another. Keys and keyseats 
are accurately machined and are capable of trans- 
mitting large forces. The machining of a keyseat in 
a part is a fairly expensive operation and keyseats 
are not put into parts just for the purpose of loca- 
ting them in fixtures. If, however, the part already 
has a keyseat, then this keyseat can be utilized for 
radially locating the part relative to a fixture. 

Keys and keyseats are used for the most part, in 
connection with circular mating surfaces, to prevent 
rotation. They are also used between parts with flat 
surfaces to prevent transverse shifting. These two 
arrangements of keys and keyseats are shown in 
Fig, 6-24. Each of them may also be utilized for lo- 
cating the part in a fixture. When used in machinery, 
a key and its keyseat serve as parts of a permanent 

Fig. 6-24. The key as a radial locator. 

assembly and are not exposed to wear. Where key- 
seats and keys are used as fixture elements, they are 
continually exposed to wear because parts are in- 
serted and removed all the time, as long as the fix- 
ture is in operation. Hence it is recommended that 
the key, at least, be made of hardened steel and also, 
if necessary, a hardened insert be provided for the 
keyseat. Keys and keyseats are usually located on 
small radii and have a tight fit; when used for radial 
locating in a fixture, they should be made with a 
sliding fit and with the closest possible tolerances. 

Dual Cylinder Locating 

Dual cylinder locating uses a flat base and two 
cylindrical locators in mating holes. This eliminates 
all six degrees of freedom and provides excellent 
mechanical stability with an accuracy which de- 
pends only on clearances in the holes and tolerances 
on the hole center distance. The interplay between 
these tolerances and clearances creates a specific 
problem for which there exists a specific solution, 
the diamond pin. 

Assume first, a rather special case where the cen- 
ter distances match so closely that their tolerances 
can be ignored. The locating accuracy then depends 
entirely on hole clearances which can be minimized 
by the use of expanding locators. The expanding lo- 
cator is shown in Fig. 6-25 where A is the bushing, 
fitting the finished hole in the work. This bushing is 
split in several different ways, either by having one 
slot cut entirely through it, and two more slots cut 
to within a short distance of the outside periphery, 
or by having several slots cut from the top and from 
the bottom, alternating, but not cut entirely through 

Fig. 6-25. An expanding locator for minimizing the clearance. 

Ch. 6 



the full length of the bushing. The method of split- 
ting, however, in every case, accomplishes the same 
object, that of making the bushing capable of ex- 
pansion so that when the stud B, which is turned to 
fit the tapered hole in the bushing, is screwed down, 
the bushing will expand. 

It should be noted that the stud actually consists 
of four different sections, the head; the tapered 
shank; a short cylinder; and the screw thread, The 
cylindrical section matches a precision bore in the 
fixture base and defines the location of the axis of 
the stud. 

In more general cases, these almost ideal condi- 
tions do not apply. There are tolerances on two 
holes, two locators, and two center distances; the 
tolerances must be adjusted to each other in such a 
manner that they leave sufficient clearance around 
each locator for any permissible dimensional condi- 
tion, and radial locating must be accomplished with 
prescribed angular accuracy. 

To illustrate the problem, consider a part with two 
holes of diameter D and center distance L ± T. As- 
sume zero tolerance on all hole diameters and on the 
center distance L in the fixture. As seen from Fig. 
6-26, it is necessary to reduce the diameter of the 
pin at the right from D to D — 27 to make it pos- 

sible for the part to be nested in all cases (with all 
center distances from L — T to L + 1% Conse- 
quently, a clearance of IT is introduced between the 
pin at the right and the hole in the part, resulting in 
an angular error 


8 = -j- radians 

The case is oversimplified by omitting most of the 
tolerances but the result would only be aggravated 
by taking all tolerances into proper account. 

It is obvious that the problem could be eliminated 
by elongating the hole in the part (Fig. 6-27). It is 
also obvious that it is not practical to make elon- 
gated precision holes in parts just to fit them into a 

Fig. 6-27. Hypothetical locating to an elongated hole. 

Fig. 6-26. The general case of dual cylindrical locating. 



Ch. 6 

fixture. Any modification to be made must be with 
respect to the configuration of the pin and it must 
permit relative motion between the hole and the pin 
in the direction of the hole's centerline while it re- 
tains a close fit between these two members in the 
direction perpendicular to that centerline. 

The Diamond Pin 

A solution along these lines is physically possible 
and technically practical because the fit between the 
hole and the major dimension on the pin must be a 
clearance fit to permit easy loading and unloading 
of the part. 

The cross section of the pin is rhombic (hence the 
name "diamond pin," see Fig, 6-28) with lengths. 
It fits into a hole of diameter D with a clearance C, 
so that 

D=A + C 

Assume first that the section terminates in sharp 

points at its upper and lower end (upper part of the 

figure). Then using the formula for a circular seg- 

ment in which -j is the chordal height and T the width 

of the segment: 

(1% £(d-C\= ™_c^cd 

W 2\ 2J 2 4 " 2 

T = y/2 CD 

Actually the pin does not terminate in points, but 
has wearing surfaces of width W (lower part of fig- 
ure) so that A becomes a diameter (the pilot di- 
ameter) and 

W+T= y/2CD 

Fig. 6-28. The geometry of the diamond pin. 

sible width W increases with D and decreases with 
center distance tolerance T. 

Various suggestions have appeared in the literature 
for the width W. The recommended value is 1/8 of 
D, with 1/32 to 1/64 inch (0.8 to 0.4 mm) as a 
lower limit. All this is now history as these pins are 
available in standardized dimensions with W= 1/3/4. 

A proposed N.U.F.C.M. standard (see Chapter 17) 
comprises sizes up to 1 inch; individual manufac- 
turer's standards go up to a 3-inch (75 mm) nominal 
diameter. The pilot diameter is available in two ac- 
curacy ranges, A and B. Pins can be installed with 
press fit, press fit with locking screw, and screw 

Jig and fixture literature occasionally recommends 
the use of two diamond pins set perpendicular to 
each other ("crossed diamond pins," see Fig. 6-29), 
The pin at A prevents longitudinal motion, the pin 
at B allows for longitudinal tolerances and prevents 

C is a measure of the angular error and is small; it is 
selected from the viewpoints of desired locating ac- 
curacy and a sufficiently easy sliding fit. T is the 
total longitudinal tolerance; it includes tolerances 
on center distances in the part and the fixture and 
the diameter tolerances and clearances on the hole 
and the pin at the other end. The width W is theo- 
retically selected from wear considerations. 

It follows from the above formula that for a given, 
or desired, tolerance T, the maximum permissible 
width W increases with hole diameter D and clear- 
ance C. It also follows that for a given hole clear- 
ance C (and corresponding angular error) the permis- 

Fig. 6-29. The use of crossed diamond pins. 

Ch. 6 



up and down motion at B. The justification appears 
somewhat incomplete since the up and down motion 
at A is not prevented. To use the crossed diamond 
pin principle would require one additional locator, 
for example, an external pin or a button, as indi- 
cated by the dotted lines, 

A fully legitimate use of a single diamond pin in 
combination with another locator is shown in Fig. 
6-30. Up and down and angular locating (not shown) 
is done by the fixture base; the diamond pin locates 
the part lengthwise, while allowing for the tolerance 
on the hole center distance a above the base. 

Fig, 6-30. One diamond pin used in combination with a flat 
locating surface. 

AJ1 dual cylinder locating systems can be designed 
for easy loading by application of the two common 
principles, p re-positioning and successive entering 
(one at a time). An illustration of the use of these 
two principles is shown in Fig. 6-3 1 . The two pins are 
shaped for p re-positioning in two different ways; 
one pin is shown with a long lead and the other is 
chamfered. The length of the actual locating sur- 
face is also different on the two pins. The part 
enters first on the long pin to the left, and is sup- 
ported and guided when it subsequently enters the 
short pin to the right. 


Fig. 6-31. Dual cylindrical locating arranged for preposition- 
ing and successive entering (one at a time). 

Typical Applications of Dual Cylinder Locating 

Dual cylinder locating is simple, reliable, and inex- 
pensive. If a part, as designed, does not have the 
two holes that are needed for locating purposes, 
such holes (sometimes named "tooling holes") can, 
in many cases, be drilled and reamed without im- 
pairing the function of the part. The same tooling 
holes can be used for locating the part in several fix- 
tures, one at a time-and even for reconditioning 
operations at some later time. 

The principle is extensively used in mass produc- 
tion such as in the automotive industry. For ex- 
ample, two holes are drilled and reamed in the pan- 
rail of the cylinder block to closer positional toler- 
ances than required for functional purposes. These 
holes serve to locate the block for all operations 
except for machining the transmission-case face on 
the end, the pan-rail face, and the head faces; these 
having been machined in earlier operations. The 
part is then entered on the conveyor in a transfer 
machine. Movable "shot pins" enter the locating 
holes in the pan-rail to locate the block at each sta- 
tion of the transfer machine. All major automobile 
companies in the United States use this system to 
machine engine blocks. Smaller automotive com- 
ponents mounted on movable fixtures (also called 

Courtesy of The Cross Co, 

Fig. 6-32. Locating holes (indicated by arrows) in a V-8 
cylinder block. Holes are for the shot pins with 
which the block is located in various machining 



Ch. 6 

"transfer" fixtures or "pallet" fixtures) are ma- 
chined in transfer machines by moving the fixture 
with the workpiece from station to station. In many 
such cases, the part is located in the pallet fixture 
on locating pins, and the pallet fixture is located at 
each machining station by means of shot pins. Ex- 
amples of these techniques are shown in Figs. 6-32 
and 6-33. 

Courtesy of The Cross Co. 
Fig. 6-33. Loading a transmission case on the pallet fixture. 
Three locating holes with adjacent bearing sur- 
faces (indicated by arrows) are machined in a 
previous operation. In the loading station, the 
transmission case is manually loaded on three 
locating and bearing points in the pallet fixture 
and the clamping straps are brought into place. 
In the following station, [he clamping nuts are 
automatically tightened to a predetermined a- 
mount of torque. 

Indexing Fixtures 

Indexing means to rotate a part to a predetermined 
angle and secure it in the new position. Very often, 
but not necessarily always, the angle of rotation is a 
simple fraction of a full circle and repeated indexing 
will finally bring the object back to the starting po- 
sition. Sometimes the word "indexing" is also used 

for a straight-line motion of a predetermined length, 
followed by a locking operation-in other words, 
"indexing in a straight line." Both types of indexing 
are used in fixture design; angular indexing is by far 
the most common. 

A primitive and inexpensive, but not very accurate, 
indexing device consists of: a fixture with a bearing 
pin that fits into a hole in the part, a number of 
markings on the periphery of the part, a target mark 
on the fixture, and a clamping device. One marking 
at a time is aligned against the target mark and the 
part is then clamped and machined. 

A part with a central bore and a number of holes 
of equal size located in a circle concentric with the 
central bore can function as its own indexing device. 
The fixture has a locator- for the bore and a hole 
with a pin targeted on the hole circle. When the pin 
is brought to enter a hole in the part, the part is lo- 
cated. This is, essentially, a case of dual cylinder 
locating. When the pin is withdrawn, the part can 
be indexed to the- next position and again locked 
with the pin. 

Schemes such as these are inexpensive to make, 
but are slow in operation and not very accurate as 
correct operation depends fully on the skill and at- 
tention of the operator. There is also no provision 
for compensation for wear. 

Most indexing operations are far more demanding 
with respect to accuracy, fast operation, and fool- 
proofing. Accuracy means two things, accuracy in 
operation, and sustained accuracy during the entire 
life of the fixture. Provisions for the satisfaction of 
all these demands must be built into the fixture. 
In addition, the fixture must possess rigidity and 
strength, and have proper locating and clamping de- 
vices for receiving and holding the part. 

The fundamental component in most indexing 
fixtures is the indexing table. It performs the fol- 
lowing functions: It receives and holds the part by 
means of locators and clamps. It rotates around an 
axis with a minimum of play and error. It carries 
the weight of the part and the load from the ma- 
chining forces and transmits these forces to the fix- 
ture base. Finally, it indexes accurately from po- 
sition to position in a manner uninfluenced by the 
natural wear on its moving parts. 

An example of an indexing fixture that satisfies 
all of these requirements is shown in Fig. 6-34. In 
this indexing mechanism one of the chief points in 
design is to prevent variations in the spacing due to 
wear on the mechanism. The fixture is so arranged 
that wear on the indexing points is automatically 
compensated for by the construction of the device; 
therefore, the provision made for its upkeep is ex- 

Ch. 6 


cellent. In addition to this feature, the design is not 
very expensive and it may be made up at much less 
cost than many other kinds of indexing devices. 
The work ,4 is a clutch gear, the clutch portion B of 
which is to be machined in this setting. As the work 
has been previously machined all over, it is necessary 
to work from the finished surfaces. 

Fig. 6-34. A typical and well- designed indexing fixture for 
milling clutch teeth. 

The body of the fixture G is of cast iron and it is 
provided with two machine steel keys at P; these 
keys locate the fixture on the table by means of the 
T-slots, and the holddown bolts Q lock it securely 
in position. The revolving portion of the fixture F 
is also of cast iron and has a bearing all around on 
the base, while the central stud C is used as a locator 
for the work at its upper end, and holds the revol- 
ving portion down firmly by means of the nut and 
collar at H. The fitting at this point is such that the 
fixtuie may be revolved readily and yet is not free 
enough to permit lost motion. A liner bushing of 
hardened steel is ground to a nice fit on the central 
stud at E and will wear almost indefinitely, while 
an indexing ring L is forced on the revolving portion 
F of the fixture, and doweled in its correct position 
by the pin V and held in place by the four screws/?. 
The work is held down firmly on the revolving por- 
tion by means of the three clamps J, these being 
slotted at K to facilitate rapid removal. 

A steel index bolt M of rectangular section is care- 
fully fitted to the slot in the body of the fixture, 
and beveled at its inner end S so that it enters the 
angular slots S and T of the index ring. Clearance is 
allowed between the end of the bolt and the bottom 
of these slots so that wear is automatically taken 
care of. A stud O is screwed into the underside of 
the index bolt and a stiff coiled spring at N keeps 
the bolt firmly in position. The pin U is obviously 
used for drawing back the bolt and indexing the 
fixture. Points worthy of note in the construction of 
this fixture are the liner bushing at E, the steel lo- 
cating ring /. , the automatic method of taking up 
wear by the angular lock-bolt M, and the spring N. 

With ample bearing dimensions and a hardened 
steel liner or liners, in the bearing, the fixture can 
operate year after year with negligible wear because 
the amount and velocity of the motion-are small, 
and the bearing is practically unloaded during in- 
dexing. Should wear ever exceed the permissible 
limits, it is a simple matter to replace the central 
stud and the liner. The same applies to the index 
ring, as it is also a separate item. The part that re- 
ceives most wear in this, as in any other indexing 
device, is the index bolt. It retains its beveled shape 
even when worn and continues to align the index 
ring properly as it closes in by the force of the 
spring. In this way, the closing; locating; and lock- 
ing action are made independent of the operator. 

The flat, beveled index bolt as here described, is 
the most efficient type that exists. Round index 
bolts are cheaper to make but less accurate in their 
operation. The axes for the index bolt and index 
holes require alignment in three directions against 
only two for the flat bolt. Round bearing surfaces 
are less resistant to wear than are flat surfaces. 
When a cylindrical index bolt is worn, it has lost its 
accuracy. A conical (tapered) index bolt (see Fig. 
8-11) can be just as accurate as the flat bolt, as long 
as it is new; as it wears, however, it forms recesses 
and loses accuracy. 

Only very large indexing fixtures for heavy parts 
require ball or roller bearings. The bearings used in 
such cases are the same types as are used in large 
precision machine tools. 

The milling machine dividing head could perhaps, 
in a sense, be considered an indexing fixture; the 
same applies to the various types of rotary tables, 
horizontal; vertical; and tilting. They are all work 
holders, and so are standard vises, magnetic chucks 
and faceplates, lathe chucks, and so on. However, 
they are designed as general purpose tools and they 
are all commercially available. For these reasons, 
they shall not be further discussed in this book 



Ch. 6 

except where they may serve as bases in actual fix- 
tures for special applications. Indexing pins with 
liners or bushings (see Chapter 17) are standardized 
and are commercially available. 

In the previous examples, the pulling of the index 
bolt and the rotation of the work were straight 
manual operations. These operations can, of course, 
also be performed by various mechanical means. 
There is another extremely simple device, which 
may even be called a trick, that can be incorporated 
into the design of an indexing fixture which permits 
rapid indexing without the need for installing ad- 
ditional mechanisms. It consists of selecting a rather 
large included angle for the bevel or taper on the in- 
dexing bolt. On a cylindrical indexing bolt, this 
angle is zero. On most ordinary indexing fixtures 
the angle is 8 to 12 degrees, so that the bolt is 
self-locking. If the included angle is made larger 
than two times the angle of friction, the bolt is no 
longer self-locking, but can be pushed back and out, 
if a sufficiently large turning moment is applied to 
the index table. Such devices in the form of spring 
stops, ball plungers and detents (see Chapter 17), 
are also commercially available. 

The drill jig shown in Fig. 6-35 was designed for 
drilling four angular holes in a brass time-fuse cap. 
(See sectional view of cap at lower part of illustra- 
tion.) The principle of this jig can easily be applied 
to other work. The jig consists of a hardened steel 
locating plate A mounted on a hardened spindle, 
which runs in a bushing that is also hardened. A 
ball bearing B takes the thrust of the spindle. At 
the other end of the spindle is an index plate C in 
which are cut four 90-degree notches. Keyed to the 
index plate, and also to the spindle, is a ratchet 
wheel D, having four teeth. A hand-lever E, which 
has a bearing and turns around a hub on the index 
plate, carries a spring pawl F that engages with the 
ratchet wheel D. The lever also carries, at the outer 
ends, two pins G that project downward, so that 
when it is pushed back and forth, the pins strike on 
the body of the jig and prevent carrying the index 
plate beyond the locking pin H. This locking pin is 
a hardened steel sliding pin, one end of which is 
rounded and engages with the notches in the index 
plate. Back of the pin, and held in place by a head- 
less set -screw K, is a coil spring J, which holds the 
locking pin against the index plate. The tension of 
this spring is just enough to hold the work from 
turning while being drilled, but not enough to pre- 
vent its being readily indexed by a quick pull on the 
indexing lever. 

The work is held in position against the locating 
plate A by the plunger L, which rests on a single, 

Fig, 6-35. An indexing drill jig operated by hand lever and 
foot treadle. 

) /2-inch, hardened steel bail that acts as a bearing 
while the work is being indexed. Plunger L is car- 
ried in a second plunger M, which is held up by a 
powerful coil spring A^. The outer plunger M is op- 
erated by a foot-treadle connected to the lever O. 
In operation, the foot-treadle is depressed and a 
piece of work is placed between the plunger L and 
the locating plate A. When the treadle is released, 
the work is held by the tension of the spring N while 
the indexing is done by lever E. The locating plate 
A has slots milled in it with a radius cutter of the 
same radius as the drill to be used. This feature, in 
connection with the lip on the work, answers the 
same purpose as a drill bushing; no other means 
of guiding the drill being necessary. The production 
record of this jig was about 4000 caps per day, 

Stability Problems 

Some indexing fixtures present stability problems. 
Small or flat parts with a short dimension in the 
direction of their axis are easily handled on indexing 
fixtures with a horizontal table. Heavy parts of 
greater axial length cannot be fix tu red with an over- 
hang, but require the equivalent of an outboard 
bearing. There are cases where an actual outboard 
bearing can be added to the fixture, but usually, this 
is an impractical solution and it is necessary to use a 
fixture provided with two trunnions supported in a 
separate cradle with two bearings, as shown in the 
following example. 

Ch. 6 



It is necessary to drill quite a number of holes in 
the casting shown in place in the jig illustrated in 
Fig. 6-36; these holes are located on different sides 
and at various angles to one another. For this rea- 
son, an indexing jig is employed. This illustration 
shows the cover A of the jig removed in order to il- 
lustrate more clearly the position of the casting, 
which is located in the jig by its trunnions. The 
main body of the jig is also supported by heavy trun- 
nions at each end, and the large disks B and C enable 
it to be held in different positions. These disks 
contain holes which are engaged by suitable index- 
ing plungers D, at each end of the fixture. 

Fig, 6-36. A large trunnion-mounted indexing jig. 

Adjustable Locators 

The term "adjustable locators" is occasionally used 
with several different meanings, and some clarifica- 
tion is therefore required. In Chapter 3 the differ- 
ence between "locators" and "supports" was ex- 
plained. "Locators" are the elements that are neces- 
sary and sufficient for full geometrical definition of 
the locating of the part; they may or may not be 
sufficient, however, for the stable mechanical sup- 
port against all the forces acting upon the part when 
it is being clamped and machined. Any additional 
elements that may be required for this purpose are 
termed "supports." 

One basic function of the locators can be described 
as the elimination of the six degrees of freedom. 
In mechanical language, this means that the locators 
bring the part into a statically determinate position 
with respect to the fixture, and any additional sup- 
port makes the position statically indeterminate. 
Any such support is said to be "redundant." 

A statically indeterminate position or system is 
not necessarily bad. The redundant supports do no 

harm if they are compatible with the part; this 
means, if they are fitted so closely that they main- 
tain contact with the part without exerting any 
force upon it. If the redundant supports are incom- 
patible with the statically determinate system, there 
are then three possibilities: 

1 . They fail to contact the part; in this case, they 
are ineffective and could be dispensed with. 

2. They lift the part off one or several of the 
locators; in this case, they assume or usurp 
the locators' function, 

3. They exert significant forces upon the part, 
and in so doing, they impose a deformation 
(deflection, distortion) within the part and 
loads (reactions) on the locators. They 
"spring" the part (if they do not bend or 
break it!). 

The various possibilities (compatibility and the 
three forms of incompatibility) are shown in Fig. 
6-37. The part is supported as a beam on two end 
supports and is, in this condition, statically deter- 
minate. The addition of a redundant intermediate 
support makes the part statically indeterminate. 
Clearly, each of the three alternative forms of in- 
compatibility is unacceptable, and redundant sup- 
ports are therefore made adjustable. The various 
designs are described in Chapter 12. 

The adjustable locators to be described in this sec- 
tion are the basic locators conforming to the 3-2-1 
principle or its equivalent. Adjustable locators are 
used for the following purposes: To accommodate 
raw parts that exceed normal or previously estab- 
lished tolerances, to adjust for dimensional changes 
within the fixture from wear, abuse, or neglect, and 
to use one fixture for more than one size of the part. 
Examples of devices for these purposes are shown 
later in Figs. 6-43 and 6-44. 

It is fundamental for the use of fixtures that the 
raw parts are dimensionally uniform within the pre- 
scribed tolerances for which the fixture is designed. 
A part that exceeds tolerances should be intercepted 
by inspection so that it does not reach the fixture. 
If it does, however, it would be rejected by the op- 
erator as soon as he finds that it does not fit proper- 
ly in the fixture. An adjustable locator should, as a 
rule, not be operated just to save an accidental or 
isolated misfit. 

Dimensional changes within a raw part may occur 
from time to time. Common causes are change of 
supply source, variations (intentional or uninten- 
tional) in foundry practice, overhaul or replacement 
of forging dies or other tools, etc. If the change 



Ch. 6 

° (m\- 




Fig. 6-37. Compatible and incompatible statically indeter- 
minate I oca tors. 

Fig.6-38. A threaded adjustable locating point. 

ly tightened up and loosened, to hold and release 
the work, when the intention is that these screws, 
when once adjusted, should remain fixed. It is not 
even possible to depend upon the locknut stopping 
the operator from using the screw as a binding screw, 
A headless screw, therefore, is preferable, as it is less 
apt to be tampered with. 

A different form for the adjustable locator of the 
screw type is shown in Fig. 6-39. s The head is hex- 
agonal and the top of the screw is rounded (crowned) 
so that it offers a regular bearing area even when the 
screw axis is slightly out of alignment due to clear- 
ance in the screw thread. The bearing area in all 

exceeds the fixture tolerances and appears to be per- 
manent, it is necessary to readjust the fixture. This 
is a toolroom operation and is followed by an in- 
spection similar to the inspection of a new fixture. 
The operator should not reset locators or other vital 
adjustable parts in a fixture. Adjustable locators are 
purposely so designed that they do not invite, en- 
courage, or facilitate adjustments "on the shop 
floor." In contrast, adjustable supports are de- 
signed for convenient and fast operation, preferably 
without the use of tools. 

Adjustable Locating Points 

The most common form of adjustable locating 
points is the set-screw provided with a locknut, as 
shown in Fig. 6-38. The screw A, is a standard 
squarehead set-screw, or, in some cases, a headless 
screw-with a slot for a screw driver; this screw 
passes through a lug on the jig, or jig wall B, itself, 
and is held stationary by a locknut C tightened up 
against the wall of the jig. Either end of this screw 
may be used as a locating point, and the locknut 
may be placed on either side. By using a square- 
head screw, adjustment is very easily accomplished, 
but unless the operator is familiar with the inten- 
tions of the designer of the jig, locating points of 
this kind are sometimes mistaken for binding or 
clamping devices, and the set-screws are inadvertent- 

Courtesy oft'. Thauiow 
Fig. 6-39. A threaded adjustable locating button with crown- 
ed head. 

screw type locators is hardened. Screw locators are 
much longer than fixed locators. They can be used 
as side and end locators without difficulty, but not 
always as base locators because of the limited verti- 
cal design space in the bottom of a fixture. 

Adjustable base locators can be designed on the 
wedge principle, The action of a wedge is mechan- 
ically equivalent to the action of a screw, but the 
wedge has its major dimension perpendicular to its 
direction of action. The wedge is, therefore, a suit- 
able device for adjustments in a narrow space. 

An example of a wedge-operated adjustable base 
stop is shown in Fig. 6-40. The base stop C is raised 
and lowered by the sliding motion of wedge A . The 

S E. Thauiow, Maskinarbejde 
Gad's Forlag, 1930) vol. II. 

(Copenhagen; G.E.C. 

Ch. 6 



Fig. 6-40. A wedge-operated adjustable base stop. 

wedge is provided with a handle B, so attached that 
it can easily be operated. It is held in place by two 
shoulder screws that are inserted through two elon- 
gated slots milted in the wedge; these screws are 
tightened after the stop has been brought up to po- 
sition. One disadvantage in using this type of stop 
is that owing to the vibration of the machine while 
in operation, the wedge is prone to slip back, caus- 
ing the stop C to drop down. Various improvements 
are possible, however, and will be described in Chap- 
ter 12, in connection with supporting elements of 
a similar type. 

The "sliding point" is another adjustable locator 
which is used extensively in fixtures. It requires 
considerable design length and must also be acces- 
sible from above or from the side. Its principal ap- 
plication, therefore, is for side and end stops. One 
design is shown in Fig. 6-4 1 , where A represents the 
work to be located; B the sliding point itself; and C 
the set-screw, binding it in place when adjusted. The 
sliding point B fits a hole in the jig wall and is pro- 
vided with a milled flat, slightly tapered as shown, to 
prevent its sliding back under the pressure of the 
work or the tool operating upon the work. This 
sliding point design is frequently used, but it is not 
as efficient as the one illustrated in Fig. 6-42. In 
this design the sliding point A consists of a split 
cylindrical piece, with a hole drilled through it, as 
illustrated in the diagram, and a wedge or shoe B 
tapered on the end to fit the sides of the groove or 
split in the sliding point itself. This wedge B is 

Fig. 6-41. A sliding point with a lock screw. 

Fig. 6-42, A split cylinder sliding point expanded by a 
wedge and a lock screw. 

forced in by a set-screw C, for the purpose of bind- 
ing the sliding point in place. Evidently, when the 
screw and wedge are forced in, the sliding point is 
expanded, and the friction against the jig wall D is 
so great that it can withstand a very heavy pressure 
without moving. Pin R prevents the sliding point 
from slipping through the hole and into the jig, 
when loosened, and also makes it more convenient 
to get hold of. In Table 6-1 are given the dimensions 
most commonly used for sliding points and binding 
shoes and wedges. 

Regardless of differences in design, all adjustable 
locators have two important features in common; 
they require tools (wrenches, screw drivers, etc.) 
for resetting and adjustment, and they can be 
locked hard. 

Adjustment for Wear 

The adjustable point locators as described in the 
previous section are essentially designed for adjust- 
ment to wide dimensional variations on raw parts 
with wide tolerances, and locator wear is not a sig- 
nificant factor. 

Adjustment for wear as well as for locator dis- 
placement from other causes such as overload, care- 
lessness, neglect, misuse, and accidental damage, is 
also required on precision locators to be used on 
parts with close tolerances. Adjustable locators of 
the screw and wedge type can be designed with a 


Table 6-1 . Dimensions of Sliding Points and Shoes or Binders 

Ch. 6 

Shoe or Binder 

Sliding Point 

Dimensions, in Inches 


2 % to 3 


1 % to 3 



2 % to 3 


2 % to 3 










Dimensions, in Millimeters 


57 to 75 


57 to 75 



57 to 75 


57 to 75 








fine adjustment ratio (fine pitch screw threads) for 
this purpose and used in drill jigs and milling fix- 
tures. Lathe fixtures present special problems as 
they do not always provide the space required for 
screw and wedge locators. They are exposed to 
wear and also risk accidental damage when mounted 
on or removed from the lathe spindle, with a result- 
ant misalignment of the fixture axis. Adjustment for 
this type of error requires certain devices for recen- 
tering of the locator section of the fixture. 

One fixture for this purpose, which may also be 
adjusted to handle several sizes of work^, is shown 
in Fig. 6-43. It is essential to be able to true this 
fixture when it is mounted on the spindle nose 
since absolute concentricity is required between the 
machined surfaces. This is accomplished by four 
adjusting screws D and a wedge pin assembly, which 
will be described later. 

The basic fixture components are the nosepiece 
B, which can be designed to fit any standard spindle 
nose in the conventional manner, and the fixture 
bodyC. A hardened steel locating ring H is mounted 

on the fixture body with a sliding fit and clamped 
in place by the socket head screws /. The workpiece 
A is located and held inside this locating ring by 
three strap clamps K. Rings of several different 
sizesare made, which can be mounted on the fixture 
body to accommodate different sizes of workpiece s. 
Whenever this fixture is mounted on the spindle 
nose of the lathe, the concentricity of the locating 
ring H should be checked with respect to the rota- 
tion of the spindle, using a dial test indicator capable 
of reading to .0001 inch (0.025 mm). If the locating 
ring does not run true, it can be adjusted by means of 
the four adjusting screws!) in a manner similar to ad- 
justing the jaws of a four-jaw chuck. When adjusting 
the fixture, only two opposing screws should be 
loosened at any one time while the other two remain 
tightened. In this way the fixture body will remain 
seated against the nosepiece while the adjustment is 
made. The fixture is ready to be used when the 
locating ring // is true within .0002 to .0003 inch 
(0.005 to 0.008 mm) with all of the adjusting screws 
D tightened. 

Ch. 6 



§ s 

3" I 

& 1 

I £ 





Ch. 6 

& E 

1 " 

a= .y 

x a 




Ch. 6 



Tightening the adjusting screws D serves to clamp 
the fixture body C securely to the nosepiece 5, and 
to locate the fixture accurately in the axial direction 
by forcing it to register against a locating surface on 
the face of the nosepiece. This is accomplished by 
the action of the wedge pin assembly, consisting oi 
a wedge pin E, a wedge-pin seat F, and a wedge-pin 
seat container G. The four wedge pins fit closely 
in the holes below the adjusting screws D. These 
holes should be tapped only to a depth that will 
allow sufficient room for the adjusting screws to 
operate. The tap drill hole should be reamed to 
size and a hard reamer should be used to remove 
any burrs in these holes resulting from the tapping 
operation. The round wedge-pin seat containers G 
are made of hardened steel and are press fit into the 
nosepiece B. An eccentric hole is drilled in the seat 
of these containers, which must be located in the 
forward position, as shown in Fig. 6-43, when the 
containers are pressed into the nosepiece. A slight 
inaccuracy in the position of the eccentric hole is 
not harmful because it is made much larger than the 
pin that is placed in this hole. This pin is pressed 
into the face of the wedge-pin seat F and it serves to 
locate the wedge-pin seat so that the bevel ground on 
the opposite face will be oriented approximately in 
the right direction. The wedge-pin seat F is a very 
loose fit in the wedge-pin seat container G to pro- 
vide it with a limited freedom of movement. The 
bevel angle on the wedge pin and on the wedge-pin 
seat as well, should be 15 to 22 degrees. Thus, when 
the clamp screws are tightened, the wedges, or bevels 
will cause a reaction of the clamping force, so that 
it will have both a radial component and an axial 
component. The radial component will hold the 
fixture body in the correct radial location and the 
axial component will hold it against the nosepiece, 
thereby providing axial location. 

The workpiece A is machined with an 80-degree 
diamond shaped insert L held in a disposable insert 
toolholder. The toolholder is held in an adapter 
that is mounted on the face of a turret on an NC 
lathe. It could also be held in a conventional 
manner on an engine lathe, or on a turret lathe 
having a cross-sliding saddle. The cutting tool is 
used to machine the faces and the major recess. 
Of compact design and built close to the spindle 
nose, this is an example of a fixture designed for 
standard work that requires accurate machining and 
where the production lots are small. Although it is 
heavy, there is so little overhang that its weight is of 
small importance. 

Another fixture incorporating the adjusting screw 
and wedge-pin principle is shown in Fig. 6-44, This 
fixture illustrates a different and more sophisticated 
clamping device, which is an embodiment of the 
floating principle. The workpiece is a bevel gear A 
and the fixture consists of two principal parts, the 
spindle nosepiece B and the fixture body C. 

The workpiece is mounted on a hardened steel 
locating ring H, which is pressed onto the fixture 
body. This ring has a clearance groove to collect 
small chips and dirt, enabling the workpiece to 
register against the locating face of the fix ture body. 

When the fixture is mounted in the lathe, the 
locating ring H must be trued with respect to the 
rotation of the spindle. This is done, as before, by 
indicating the locating ring with a "tenth" dial test 
indicator and adjusting the adjusting screws D until 
the locating ring is true within .0002 to .0003 inch 
(0.005 to 0.008 mm). The wedge clamp assembly 
consists of the following parts: E wedge pin; F 
wedge-pin seat: and, G wedge-pin seat container. 
This assembly will cause the fixture body C to be 
held firmly against the nosepiece B as described for 
the fixture shown in Fig. 6-43. 

The method of clamping consists of the use of 
three strap clamps L, a clamp operating screw 7, and 
a floating collar K. The three clamps are placed 120 
degrees apart and have slightly oversize holes through 
which the clamp retaining screws M pass. These 
screws have a bail surface on the underside of the 
collar corresponding to a similar depression in the 
clamps themselves. A bronze or steel bushing / is 
pressed into the fixture body C, and is threaded with 
a coarse-pitch thread which corresponds to that on 
the clamp operating screw J, After the clamps L 
have been swung into place on the ring gear, a few 
turns of the damp operating screw tightens all three 
of the clamps against the ring gear A through the 
action of the spherical floating collar K, which bears 
against the inner sides of the clamps. 

Where high production is required, a machine 
equipped with a rotating pneumatic cylinder is used. 
In this case the threaded bushing / would not be 
used. The screw J would be threaded directly into 
an operating rod that extends through the inside of 
the lathe spindle, which is then attached to the 
pneumatic cylinder. The pneumatic cylinder actuates 
the operating rod which moves the screw J forward 
to clamp the workpiece. However, on lathes that 
are not equipped with a pneumatic cylinder, the 
arrangement shown in Fig. 6-44 is very satisfactory. 

Loading and Unloading 


Entering the Part 

The complete process of fixturing is comprised of 
loading, machining, and unloading; the loading 
operation consists of entering and locating the part 
and clamping it; the unloading, of releasing and re- 
moving the part. Each phase has its problems. 
Entering involves manual handling and requires 
space. Convenient manual handling depends on 
weight and balance. Light parts are handled by the 
operator's two fingers or one hand; heavier parts 
require two hands or, in more extreme cases, a hoist, 
crane, or conveyor. Well-balanced parts require lift- 
ing and lowering only; an unbalanced part, having 
its center of gravity at some distance from its mid- 
point, also requires a steadying effort which makes 
it increasingly difficult to keep the part level during 
lifting and lowering. 

Space must then be provided inside the fixture 
for the part, fingers, a hand, possibly two hands 
(and knuckles!), or two hands and arms. For 
heavy parts there must also be clearance from the 
machine tool to allow the operator to lean over 
the fixture, or to admit the load cable from the 
hoist or crane. Although these factors may appear 
trivial, they are quite serious and it is a common ex- 
perience that space always looks larger on a drawing 
than in reality. 

Locating the Part 

Locating means bringing the part into positive 
and, correct contact with the locating points or 
surfaces. Chips and dirt on a locating point pre- 
vent direct contact at that point, but accumulations 
in other places in the fixture may well cause such 
misplacements or misalignments that the part can- 
not be properly located. Other causes of insufficient 
contact are burrs, part irregularities beyond pre- 
scribed tolerances, jamming, and friction. These 


adverse factors can be directly and indirectly con- 
trolled by the fixture designer who should provide 
means for chip cleaning and for visibility at the 
locating point. 

Apart from these considerations, there is no 
further problem encountered in locating when the 
conditions are equivalent to those shown in Figs. 
3-1 d and 4-1, Locating is done in three consecutive 
steps. First, the part is set on the base; second, it is 
moved to contact with the side stops; third, it is 
moved to contact with the end stop. Next, the 
clamping pressures are applied. A basic and 
characteristic feature in these simple examples is that 
each locating step is not interfered by, and does 
not interfere with, any other locating step. One 
result, thereof, is that the individual phases in 
locating are not sensitive to the direction of ap- 
proach. Assume the part is tilted while it is lowered 
to the base. It then contacts first one of the three 
points (or one corner), levels off, contacts the 
second point (or corner), levels off on the axis 
through these two points (or the edge between the 
two corners), and comes to rest on all three points 
(or on the bottom surface). If it is still misaligned 
with respect to the side stops, it contacts one side 
stop first, then aligns itself to contact with the 
second side stop. 

These observations lead to the basic and very 
general rule that locating should be done on only 
one surface at a time, where possible. 

Correct and incorrect Loading 

Stressing that the part be brought into correct 
contact with the locating surfaces may seem un- 
necessary, but it is not. Any part that has been 
machined when located in an incorrect position is 
lost, and so is the labor that has been expended. 
Design steps taken to prevent incorrect loading are 





'foolproofing," or "mistake-proofing" the 

Symmetry Considerations 

Correct and incorrect loading are associated with 
symmetry and asymmetry in the part configuration. 
With reference to Fig. 7-1 , planes of symmetry (or 
asymmetry) are denoted^, BB, and CO the corres- 
ponding perpendicular axes (sometimes, but not 
necessarily always, axes of rotational symmetry) are 
denoted X., Y, and Z. A completely symmetrical 
part; that is, a part containing three planes of 
symmetry, can be loaded in a fixture in four dif- 
ferent orientations. From an initial position it can 
be turned 180 degrees around the three axesjf, Y, 
and Z, respectively. In other words, it can be turned 
end-for-end and upside down, and there are no 
orientations other than these four. Apart from any 
surface markings there is no discernible difference 
between the four positions, and any machined con- 
figuration applied to the part will produce the same 
end result. Every position is a correct position and 
incorrect machining is simply not possible in this 
case, regardless of how the part was loaded. 




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x - 

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Fig, 7- 1 . A part with three planes of symmetry. 

A completely asymmetrical part, fully nested, will 
normally be able to enter the fixture in one position 
only, the correct one; and is, therefore, always cor- 
rectly machined. The possible exceptions are if the 
configuration of the nesting points and surfaces con- 

tain some degree of symmetry. AM other cases lie 
somewhere between these two extremes. Two 
important examples will now be analyzed. 

In Fig. 7-2 a part having two planes of symmetry, 
AA and CC is shown. Also shown are the three 
principal axes X, Y, and Z, in the initial position. 
Two sets of surfaces, namely, the two pads on the 
top side and the right-hand face, are to be machined 
and a hole will also be drilled, as shown in view b. 
To assist in identifying the position and orientation 
of the part, one surface has been labeled A and it is 
called the TOP SIDE SURFACE; another surface 
has been labeled B and is called the FRONT SIDE 
SURFACE. This part is shown in four different 
positions in Figs. 7-2 b, c, d, and e. If no correc- 
tive action is taken, the part may be assumed to 
enter the fixture in any of the four positions. The 
need for corrective action is evident from an exam- 
ination of the four illustrations. In Fig. 7-2 b, the 
part is in the initial position; the intended correct 
position for entering the fixture. The surfaces 
machined are the correct surfaces, and the hole is 
in the correct position. 

The part, in Fig. 7-2 c, is rotated 180 degrees 
around the Y axis. Notice that the final configura- 
tion of the part will not change when it is machined 
in this position; this is the result of symmetry on the 
A A and CC planes. 

In the position shown in Fig. 7-2 d, the part has 
been rotated 180 degrees about the X axis from 
its initial position. When the hole is drilled and 
the two surfaces are machined with the part in this 
position, the relationship of the hole and the 
machined surfaces on the pad will be incorrect. 
This is shown in the lower illustration, which shows 
the front side surface. When this view is compared 
to the front side surface in Fig. 7-2 b, it is readily 
seen that the machined pads are on the wrong side. 
The part in Fig. 7-2 e has been rotated from its 
initial position 180 degrees around the Z axis. 
Again, the part configuration will be machined 
incorrectly, which can he seen by comparing the 
front side views in Figs. 7-2 b and 7-2 e. The 
machined pads are again on the wrong side. 

An examination of the figures shows, that out of 
the four possible part positions within the fixture, 
there are only two positions (namely views b and c) 
in which the surfaces can be machined to their cor- 
rect relative positions. This observation serves to 
illustrate a fundamental rule, not generally recog- 
nized. There exists a class of operations that is 
permissible, even with the part in a prohibited 
position. The criteria for this class of operations 
are that they produce surfaces which consist entirely 



Ch. 7 

— — ~T 1 L K >OP SIDE 






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EU i? 




Cl/TTER NO. 2 



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Fig. 7-2. A part with two planes of symmetry. 

Ch. 7 



of straight line generatrices perpendicular to the 
nonsymmetry BB plane (satisfied for the end sur- 
face, the two pad surfaces, and the hole), and 
that they maintain symmetry with respect to one 
of the planes of symmetry, either the A A plane 
(satisfied for the two pad surfaces), or the CC 
plane (satisfied for the end surface and the hole). 
A part with only one plane of symmetry radically 
changes its position configuration relative to the 
fixture, with each of the three possible 180 degree 
rotations, and, normally, these three positions are 
prohibited. There also exists, however, a class of 
machining operations that will produce correct sur- 
faces with two different positions of the part in 
its fixture. The part shown in Fig. 7-3 has only 
one plane of symmetry, plane CC, It is to be drilled 
through and machined on the two pads. With the 
part in its initial position, the machined configura- 
tion, which is the correct one, is shown in Fig. 7-3 b. 
When rotated around axis X, the part is still cor- 
rectly machined, as shown in the view of the front 
side surface in Fig. 7-3 c. However, if the part is 
rotated around axis Y (see Fig. 7-3 d) or around 
axis Z (see Fig. 7-3 e), the hole is drilled in the 
wrong place. The criteria for the class of operations 
which can produce a correctly machined part from 
more than one part position within the fixture are 
that they produce surfaces which consist of straight 
line generatrices perpendicular to one of the non- 
symmetry planes, either the A A plane (satisfied for 
the pads), or the BB plane (satisfied for the pads 
and the hole), and that they maintain symmetry 
with respect to the one and only plane of symmetry 
CC (satisfied for the pads and the hole). 

Fool proofing 

Despite the literally endless variety of possible 
asymmetries found in part configurations, a syste- 
matic classification and the formulation of some 
widely applicable rules can be provided. With such 
rules and some practice, the fixture designer is able 
to quickly spot and utilize existing possibilities 
for foolproofing and to create them where required , 
by additional modifications in the fixture design. 

Foolproofing is required for parts with at least 
one asymmetry. A completely symmetrical part 
needs no foolproofing; No matter how it is in- 
serted into the fixture, it presents to the eye, and 
to the cutting tool, identically the same configura- 
tion and, no matter where the machining cuts are 
taken, relative to the planes or axes of symmetry, 
the machined parts come out with identical shapes. 
Foolproofing is needed when the part, in addition 

to one, two, or three planes of symmetry, also pre- 
sents one or more irregularities. Many workpieces, 
particularly components of machinery, fail into 
this category. The body of the part may be essen- 
tially symmetrical and of substantial dimensions, 
which makes it suitable for locating and clamping. 
The irregularities can be convex (going out) or 
concave (going in). They disturb the symmetry, 
and it is these irregularities that are utilized for the 
purpose of foolproofing. Before going into the 
details of the subject, a few simplified, but typical, 
examples will be shown for orientation. 

A two-times symmetrical part with a projecting lug 
on one end, as shown in Fig. 7-4 a, can be confined 
within a box with a cut in the end wall for the lug. 
If the cut in the end wall cannot be tolerated, the 
vacant space in the box adjacent to the lug can be 
taken up by a blocking, as shown in Fig, 7-4 b. 
With two end lugs, as shown in Fig. 7-4 c, the 
blocking can be located in the space between the 
lugs. A cylindrical hollow boss with a plain arm 
can be centered on a mandrel with the arm located 
in a straight slot in the fixture wall. If the arm is 
formed as a bracket with a T-section, as shown in 
Fig. 7-5, the contour of the slot must match the 
contour of the bracket. A straight slot of constant 
width would not prevent the part from entering 
upside down. A part with an asymmetrically lo- 
cated, downwardly open cavity, as shown in Fig, 
7-6, is very simple to foolproof. All that is required 
is a blocking on the fixture base, while an upwardly 
open cavity would require a blocking which extends 
downward from above, and therefore must be 
carried by a removable part of the fixture. 

Locating elements for foolproofing can be a part 
of the fixture base, the cover, or the clamps. In 
special cases, it may be necessary to use separate 
movable parts for this purpose. Such parts may 
simultaneously serve as centralizers (see Chapter 9), 
When possible, they should be built right into the 
fixture base, so that they take effect immediately as 
the part is entering the fixture. If placed in a 
movable part, they cannot detect an incorrect load- 
ing until after the part has been introduced, and 
additional time is then required to unload, correct, 
and reload. 

in most cases the foolproofing requires simple 
means and is done at little additional cost. It very 
often happens that the major body of the part, 
including its natural locating surfaces, contains one 
or several degrees of symmetry, and the asymmetries 
are confined to small areas at isolated locations. 
In such cases the locating is performed according to 
general rules supplemented by the necessary fool- 



Ch. 7 





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Fig. 7-3. A part with one plane of symmetry. 

Ch. 7 













Fig. 7-4. Foolproofing a part with projecting lugs of simple 

proofing elements. Localized asymmetries are 
convex or concave. When convex, they take the 
form of projections, such as arms, bosses, brackets, 
ribs, etc.; when concave, they may be recesses, de- 
pressions, notches, slots, holes, perforations, or 
cavities of other shapes. 

3 kIU^\(^ 

Fig. 7-5 Foolproofing a part with a contoured bracket. 

Projections on the part can, in most cases, be 
located or embraced between projections in the 
fixture, two at a time, forming a fork. The fool- 
proofing elements may be blocks or pins. They can 
be given wide clearance against the part because they 
are not locators in the strict sense of the word; 
all they do is select one out of two or more 
possible orientations, each of which is closely de- 
fined by the actual locators. From the viewpoint of 
simplification, economy, and efficiency it is, of 
course, desirable if the real locators can also do the 
foolproofing, or participate in it. This happens 
when a locator is made to serve as one of the two; 
prongs of the fork that embraces the projection, 
the other prong consisting of an additional block or 
pin. However, instead of constructing a fork to 
embrace the projection, the same result can be 
accomplished by a cavity in some part of the fixture 
such as the wall, a bracket, or a rib. 

Reversing the principle of a fork embracing a 
projection, constitutes the "blocking" principle; 
no constraining means are provided for the pro- 
jection at its correct position, but every incorrect 
position is blocked. The blocking principle is often 
simpler to apply than the principle of the embracing 



Ch. 7 

Asymmetries in the form of cavities are simpler to 
handle than projections because it takes only one 
block or pin to mate a cavity, but two to make a 
fork. A simple and very common case is where the 
cavities are a group of holes in a flat surface. The 
part is effectively located but not necessarily fool- 
proofed by two pins mating with two holes. If the 
holes are of different diameters, the locating is 
foolproof. If not, it may be possible to provide one 
extra hole of a different diameter for the double 
purpose of locating and foolp roofing, or the de- 
signer may find it possible to make an existing 
hole oversize, for the same purpose. Asymmetries, 
whether projections or cavities, also may be located 
essentially on a horizontal contour, in a vertical 
orientation, or tangentially; that means an asym- 
metrical or otherwise irregular spacing of the pro- 
jections or cavities on a circle. 

The example shown in Fig. 7-7 is a cylinder block 
with asymmetries (the flange contour and the hole 
pattern) in a horizontal plane. The locating elements 
between the drill jig A and the surface of the upper 
flange, and the side stops and clamping screws on 
the two long sides, have symmetry. But if no further 
steps were taken the jig could nest on the block in 
two opposite positions, and one of these must be 
prohibited. This is accomplished simply by carrying 
the end stop B, sufficiently close to the end lug C, 
so that It cannot nest on the triangular extension D, 
on the opposite end. 

shown in Fig. 7-8, where it is required to drill and 
countersink the hole in the arm. Even without the 
countersink, a skilled mechanic would not slide the 




Fjg. 7-8. Foolproofing by means of the bracket configura- 

part on in the wrong position and attempt to drill 
it without support. In the case shown, the part can 
swing into position under the bracket with the drill 
bushing only when it is first correctly located on 
the locating pin. An analogous example is shown in 
Fig. 7-9, where a hole in the arm can be located on 

Fig. 7-7. Foolproofing an asymmetrical part by means of an 
asymmetrically located block. 

Parts with vertical asymmetries are very common 
and are also frequently associated with surfaces of 
revolution. Perhaps the most dangerous configura- 
tion is one with an asymmetrical arm projecting from 
a boss with a cylindrical bore, since the bore can 
slip on a cylindrical locator in two opposite posi- 
tions. A simple and typical case of this category is 

Fig. 7-9. Foolproofing by means of a step on the cylindrical 




the pin without interference with the drill bushing 
only if the part is held in the correct position. 

The part shown in Fig. 7-10 has its boss centered 
between a lower and an upper conical locator. It can 
enter the lower locator only when the arm coincides 
with the notch provided in the cylindrical wall, and 
in no other position. 

Fig. 7-10. Foolproofing by means of a notch in the wall 
of the drill jig. 

Parts with radial locating and tangential asym- 
metries are shown in Figs. 7-11 and 7-12. In each 
case the part is centered on its axis, clamped with a 
knob K, and radially located by a pin X, in a slot. 
In Fig. 7-11, a hole is to be drilled opposite flat,4. 
Foolproofing is done by the pin B, set tangentially 
to flat A. For radial locating, this arrangement 
would be the poorest possible, as it permits a small 
but finite tangential motion. However, the actual 
locating is done by the pin X, which can be made to 
fit in a machined slot with close tolerances. 

In Fig. 7-12 the hole is to be drilled opposite a 
projecting bracket A, The part is clamped by 
means of a hinged clamp C, with a knob K, The 
clamp has two lugs; a slot in the lower lug forms it 
into a fork F, matching the bracket A when it is 
in its correct position. The damp, including the 
lugs, blocks all other positions of the bracket. 

Examples with asymmetrical cavities are shown in 
Fig. 7-1 3. A blind hole is to be drilled on the same 
side as the cavity or on the opposite side. In each 
case the foolproofing is done by means of a small 
block that matches but does not have to fit closely 
into the cavity. With the hole and the cavity on 

Fig. 7-11. Foolproofing by means of a pin. 

the same side (Fig. 7-13a), the block is mounted on 
the underside of the hinged leaf of the drill jig; with 
the hole and the cavity on opposite sides (Fig. 
7-1 3b), the block is mounted on the base of the jig. 
In each case, incorrect loading is prohibited. 


Fig. 7-12. Foolproofing by means of a slot in the hinged 



Ch. 7 

With vertical loading and unloading, the lifting is 
usually a little more difficult than the lowering. It 
is a matter of getting the proper grip or hold on 
the part, which applies to very heavy parts as well 
as to very small parts. Once this is understood, the 
operator must always make sure that there is enough 
clearance inside the fixture for getting the proper 
grip on the part. Clearances must also be checked 
against the possibility that they have caught chips 
that can bind between part and fixture, when the 
part is to be removed. 

Fig. 7-13. Foolproof ing by use of a cavity in the part. 

Most cases of asymmetry are simple to handle 
by application of the general rules and illustrative 
examples mentioned, but there are some exceptions 
requiring different solutions and one of these is the 
case of punched parts. Even with a symmetrical 
contour, such parts contain one asymmetry, namely, 
the burr, A well-designed fixture will provide burr 
clearance but requires also that the part enter with 
the correct up and down orientation. The solution 
is to provide an asymmetrically located hole in the 
part and a matching pin in the fixture. The hole 
is punched simultaneously with the part and is 
therefore inexpensive, and the punched part can 
now only be located with the burr in the right place. 

Removal Problems 

The part removal operations, after machining and 
release of clamps, are nothing more than the re- 
versal of the entering and locating operations, and 
no serious problems are encountered here, provided 
the part is identically the same as when it entered 
the fixture (less the metal removed). Although this 
proviso might be considered trivial, there are two 
very real conditions that can cause trouble if they 
are not given proper consideration— new burrs and 
warping due to heat and relief of residual stresses. 

Burred Parts 

As explained later, in Chapter 8, two types of 
burrs, the major burr and the minor burr, are formed 
in a machining operation on ductile materials. 
Burrs along external surfaces (see Fig. 8-2a and b) 
present no problem as they are out in the open; 
nor do burrs on drilled holes (see Fig. 8-2cJ if the 
part is lifted out. However, burrs from any opera- 
tion, particularly from drilling, do present a problem 
if they are formed on a surface which has to slide 
with a narrow fit in or on a mating locating surface. 
The most common case is a part with a cylindrical 
surface, to be drihed perpendicular to the cylinder 
axis. Drill jigs for this operation are shown in 
Fig. 7-14. 

The part shown in diagram a is a short shaft, 
located in a cylindrical bore in the drill jig. The 
hole is a blind hole and only a minor burr is formed 
adjacent to the drill bushing. The body of the drill 
jig has a keyseat which provides clearance for the 
burr to form and allows it to slide out. The body 
of the drill jig can be completely machined, including 
the cutting of the keyseat, before the end plug is 
welded on. 

The part shown' in diagram b is a bushing with 
a hole through on a diameter. It it located on a 
plug. The plug has a hole to clear the drill, and is 
machined with upper and lower flat surfaces which 
provide clearance for the major burr on the upper 
hole in the part, and the minor burr on the lower 

The part shown in diagram c is a stepped shaft 
with one through hole. The drill jig is machined 
from the solid. Clearance for the minor and major 
burrs is provided by two key seats inside the bore in 
the jig. Two holes A are drilled to provide end 
clearance for the cutting of the keyseats. Hole B 
is used to clear the drill. 

Burr problems are also encountered in milling 
operations, particularly those involving slotting 
operations. An example is shown in Fig. 7-15. The 

Ch. 7 



operation is to cut a slot lengthwise from one end of 
part A , a cylinder. The part is located on a mandrel 
B and is clamped by means of a large washer C. 
The mandrel has an outboard support D, which 
is removable for loading and unloading. The milling 
operation is climb milling (down milling) so that 
the work is held against the mandrel. A slot E is 
machined in both mandrel and washer to provide 
clearance for the cutter. At the same time it provides 
clearance for the major burr which is being formed 
on the inside of the<part. 


Despite all the fixture designer's care and fore- 
sight, it is not always possible to ensure free and 
easy removal of the machined part and a mechanical 
ejector then becomes a necessity. A warped and 
binding part is not the only Justification for the 
use of an ejector; in fact, the ejector should enjoy 
much wider publicity in the literature and much 

n \T7Ttfk 

l-'ig. 7-14. Drill jigs with bun clearances. 

Fig. 7-1 S. A milling fixture with a slotted mandrel for burr 

wider use in industry than is presently the case. 
The ejector is not a convenience but an economic 
asset. It reduces the time for removal of the part. 
As an example, it took 0.20 minute to grip a part, 
lift it out of the fixture and place it in the tote pan. 
With an ejector used for lifting, the same operation 
was reduced to 0,08 minute. This time saving may 
appear small; however, for large-volume production 
with short duration operation cycles, every saving 
represents a significant percentage of the total. 
By eliminating the need for finger and hand space 
for gripping the part, the ^ejector permits a reduction 
of the overall dimensions- thus the cost-of the 
fixture. The ejector pins can be located to suit 
the part configuration so that side-heavy parts are 
lifted free and clear, in perfect balance. In this 
way, ejectors eliminate inconvenient and awkward 
hand manipulation and reduce operator fatigue. 
There are several reasons why parts may bind in the 
fixture. It may be from distortion during machin- 
ing, as previously discussed, or it may be because the 
part has a locational interference fit (class LN 2 
or 3) against the locator to ensure close tolerances. 
The part is tapped down in place when loaded into 
the fixture; removal requires the application of some 
force, but is easily done with ejectors. 

For maximum time saving, the operation of the 
ejector can be automatic and coupled with the 
release of the clamp. The combined mechanism 
can be powered hydraulically or by compressed 

Ejector Details 

Ejectors are inexpensive since there is no need for 
close tolerances; most machine work is by turning 



Ch. 7 

Fig. 7-16 {Left). A single ejector. Fig. 7-17 (Right), A multiple ejector. 

and (for hardened parts) cylindrical grinding; and 
holes are finished by reaming. Details that come in 
contact with the part are usually made from 
hardened tool steel or from a case hardening steel. 
If the part surface must be protected from scratches 
or other markings, the ejectors may have contact 
heads made of copper, brass, or aluminum. 

It is a prerequisite for the use of ejectors that the 
locators must be designed jam-free, as previously 
explained in Chapter 6. Since ejectors are moving 
parts, their bearing surfaces must be protected 
against dirt and chip fragments. In most cases 
ejectors are shielded against chips by the part itself; 
if necessary, shields or seals also can be installed, as 
described at the end of Chapter 8. 

The basic elements in an ejector system are the pin 
and the spring, A single ejector is shown in Fig. 
7-16. The ejector pin A is manually operated by 
means of knob B, and is returned by spring C 
acting on the shoulder D, which also stops it in the 
return position. The pin is supported at E and F, 
The distance between the two supports must be 
several times the diameter, for jam-free guidance; 
and to provide, at the same time, the necessary 
space for the spring. 

If the part has a central hole, a multiple ejector 
must be used with two or more ejector pins. The 
ejector shown in Fig. 7-17 has two pins A, fast- 
ened to a flange on knob B. The return spring C 
is centrally located and acts on the flange. The 
arresting shoulders D are parts of the pins, which also 
provide the inner bearing surfaces E, while the 
outer bearing surface F is in a bore in the knob. 
Other arrangements are also possible. Each pin could 
have its own two bearing surfaces and its own spring. 
The simplest means of operating an ejector is the 
hand-operated knob, as shown, but it requires that 
the ejector be arranged in the side of the fixture. 

When ejection is in the upward direction, the knobs 
are inaccessible for direct hand operation, and a 
lever must be used. The example shown in Fig. 
7-18 is a single ejector. The design is simplified 
by the absence of a spring as the pin will retract by 
its own weight. 

In the multiple ejector shown in Fig. 7-19, the pins 
A are carried by a ring B, and the lever C is forked. 

Fig. 7-18 (Top). A lever-operated single ejector. 

Fig. 7-19 (Bottom). A lever-operated multiple ejector. 




The ring with the pins is returned by means of 
springs D, Pins and springs must be evenly spaced 
on the ring; on large diameters there should be at 
least three pins. Two springs are theoretically 
sufficient; however, three are recommended. 

Small parts can be automatically ejected by direct 
spring pressure. Two arrangements for this purpose 
are shown in Fig. 7-20 a and b. In diagram a the 
part is located against the fixture base by means of 
clamps; the ejector pin A is forced down flush 
with the base when the part is loaded. In diagram b 
the defining surfaces B are above the part which 
is lifted into proper position by the force from the 
ejector spring. In each case the part is automatically 
ejected as soon as the clamps are removed. 

heavy part, it must also be designed so that it stays 
in the elevated position under load. In this way the 
operator has both hands free for removing the part. 
This is automatically accomplished (the screw is 
self -locking) when the helix angle of the thread is 
less than the angle of repose (the angle of friction, 
8'/j to 14 degrees, corresponding to fJ. = 0.1 5 - 0,25) 
and may require the use of a relatively large pitch 

Ejectors can also be cam operated or wedge 
operated, which is particularly applicable to fixtures 
for multiple parts, A representative example is 
shown in Fig. 7-22. The fixture holds four parts, 
and the four ejectors are operated by one push-rod 
A, with four milled notches, tapering on one end. 

Fig. 7-20. Examples of spring-actuated ejectors. 

Spring-operated ejectors, however, are insufficient 
for heavy parts and for parts that bind on the 
locators. In such cases, screw ejectors are used. 
An example is shown in Fig. 7-21. The screw A has 
a handle B, Since this handle must project through 
a window in the wall of the fixture base, it can 
only be operated through a small fraction of a 
revolution (30 to 45 degrees) and it is, therefore, 
necessary to use a screw thread with a large lead, 
which in turn, may require a double- or triple-thread 
screw. When a screw ejector is used for lifting a 

Fig. 7-21. A screw-operated ejector. 

The pins Bare bored and slotted to receive and guide 
the ejector pins C. The four ejector pins are 
operated simultaneously by pushing in rod A . 

The principle also can be modified in various 

1 . Two (or more) pushrods can be operated by one 

2. The ejector pins can operate on one large part 
instead of on individual small parts 

3. The push -rod, or rods, can be operated by a 
lever to provide a greater lifting force with normal 
operator effort. 

An extremely simple, almost primitive, but very 
effective yet inexpensive ejector system is the 
following: The necessary number of ejector pins 
are fastened into a plate in the desired pattern, and 
the plate is then clamped to the machine table. The 
system is only applicable to drill jigs so small and 
light that they can be lifted without undue effort. 
Holes are drilled in the bottom of the drill jig 
corresponding to the pattern of the ejector pins. 



Ch. 7 



Fig. 7-22. A wedge-operated ejector for a multiple-part fixture. 

When the drilling operation is completed, the opera- 
tor opens the clamps so that the part is free, then 
lifts the jig and lowers or forces it down over the 
pins until the part has been pushed free of the jig. 

Loading of Large and Heavy Parts 

To facilitate handling when loading large heavy 
parts, the hard physical work should be done in a 
conveniently accessible space unencumbered by the 
machine tool. This is frequently accomplished 
through the use of dual fixtures set on opposite 
ends of a machine tool tabic, or with one or several 
fixtures mounted on an indexing rotary table, but a 
means can also be incorporated in the design of a 
single fixture. General rules for the design cannot 
be formulated, but a few representative examples 
of such devices, which take many different shapes, 
will be shown. 

The fixture has a movable receiver, which is moved 
to the outer station for loading and unloading and 
returned to the actual fixture station for machining. 
Being part of the fixture, the receiver must have 
devices for accurately locating itself when it is back 
in the fixture station, firmly supporting it and, 
if necessary, locking it in that position. 

Receivers, Sliding or Rotating 

When the weight of the part still permits it to be 
manually moved on a smooth horizontal surface, a 
receiver may not be needed, but the fixture base 
can be extended sufficiently outside of the machine 
headstock area to allow the part to be conveniently 
set off and then pushed into the fixture space as 
shown in Fig. 7-23. 

Rotating or swinging receivers may take many 
forms and may perform additional functions within 
the fixture. The receiver R shown in Fig. 7-24 
is actually a swinging drill jig. With the receiver in 

Z=P-^ \^J 

Fig. 7-23, A fixture with an extended base for easy removal 
of a heavy part. 

Ch. 7 



Fig. 7-24. A drill jig with a swinging receiver. 

the open position, the part is clamped on by means 
of clamps C, the receiver is swung back and locked 
in position, and drilling is done through the bush- 
ings in the receiver. The fixture shown in Fig. 
7-25 is also a drill jig. The receiver R carries a 
central locator /,. With R in the outer position, the 
part is located over I and clamped by means of 
clamps C. R is swung back and the drilling is done 

through drill jig bushings mounted in the stationary 
bracket B. The same principle is frequently used in 
the design of broaching fixtures. 

A somewhat different use of the receiver principle 
is shown in Fjg, 7-26. For small parts, the problem is 
not weight, but quantity, and the receiver functions 
here as a prepositioner for a surface grinder with a 
magnetic chuck or faceplate C. The use is therefore 
limited to magnetic materials. The receiver R is 
a plate, supported in trunnions and equipped with 
permanent magnets. With the magnet side up 
(diagram a), R is loaded then rotated 180 degrees 
and lowered onto the magnetic chuck (diagram b). 





T I I I I I I . ^-^ . 

1"%. 7-25. A drill jig with a swinging receiver. 

Fig. 7-26. A combined receiver and prepositioner. 

The permanent magnets carry the weight of the 
parts, but the stronger magnetic chuck, when 
switched on, pulls the parts down to its own sur- 
face in position for grinding. The receiver, now 
empty, is returned to its loading position. It is 
also a time-saving device in that it permits the loading 
of a new batch of parts during the grinding of the 
previous batch. 



Chip Problems 

Types of Chips 

Machining cast iron, bronze, and other brittle 
materials produces crumbling chips and a great deal 
of dust. Steel and other ductile materials produce 
several types of chips. A single-point tool, cutting 
at high velocity, as most carbide tools do, produces 
continuous chips which are long, usually curled 
into a helix of some sort, but sometimes snarling 
and bundling together, uncontrolled. With the use 
of a chip breaker on the tool, the chip flow is 
brought under control. 

Chip breakers, as a rule, are not used on high-speed 
steel tools as the cutting speeds and the volume of 
chips produced are much lower; the chips are of the 
discontinuous (segmental) type and contain numer- 
ous slip planes and cracks so that they are brittle, 
and easy to break. Chips take up a much larger 
volume than the metal from which they were 
formed. Small chips require at least three times the 
original volume; snarling and loosely wound helical 
chips may occupy up to 20 times the original 

A twist drill is essentially two single-point fools: 
the lips-set at an angle, the point angle; and con- 
nected across the center by a short edge-the chisel 
edge. The angle across the chisel edge is a large, 
obtuse angle, so that the chisel edge removes metal 
by a combination of extruding and negative rake 
cutting, thereby generating a large thrust force. The 
chips produced by the chisel edge are small and 
curling and in themselves present no problem. The 
two lips produce the same chip types as single-point 
tools except that the chip is formed and fed into 
the confined space of the flutes of the drill. This 
eliminates the possibility of large diameter, helically 
wound chips and reduces the chip types to straight 
chips; tightly wound helices; and short, or crumbling, 
chips. With coarse feeds, the first two types have 
sufficient rigidity to travel through the flute. With 


fine feeds, the chips may snarl and pack the flutes. 
Therefore, an understanding of the behavior of drill- 
ing chips is essential to the fixture designer for the 
correct axial placement of drill bushings. 

A face mill is essentially a plurality of single-point 
tools set in a circle and produces the normal chip 
types with the limitation that no chip can be longer 
than the path of the cutter tooth across the work. 
Most other types of milling cutters produce short 
chips since the path of the cutter tooth within the 
material is short. In ductile materials the chips are 
tightly rolled. Long side mills with helical teeth (slab 
mills) produce stiff, needle-shaped rolled chips as 
long as the width of the work. 

Burr Formation 

Burrs are always associated with -the machining 
of ductile materials. Truly brittle materials do not 
produce burrs, but form crumbled and broken edges. 
This is a problem of another kind which certainly 
enters into the sequencing of operations, but hardly 
into the fixture design itself. 

Burr formation is a result of the stresses in the 
metal caused by the pressure exerted by the tool. 
Consider a small element at the cutting edge and 
well within the metal (point A in Fig. 8-1). The 
element is loaded and stressed by the pressure from 
the tool but is also backed and supported by the 
parent metal from behind and from two sides. A 
similar element (point B) bordering upon the free 
metal surface has less support from behind and no 
support from one side. Consequently, it will try to 
escape from the pressure in the direction of no 
resistance and in doing so it forms a burr. Burrs 
develop at the two points B and C where the edge 
of the cutting tool intersects the metal surface. A 
larger burr, the major burr, forms at point B on the 
leading edge (the side-cutting edge in cutter terminol- 
ogy); and a smaller burr, the minor burr, forms at 

Ch. 8 



Fig. 8-1. The mechanics of burr formation, 

point C on the trailing edge (the end-cutting edge). 
No rules can be given for the dimensions of the 
burrs, except that larger burrs are developed with 
coarser feeds, lower cutting speeds, and smaller rake 
angles on the tool— and in softer and more ductile 
materials. The major burr develops ahead of the tool 
and is removed with the chip at the next passage of 
the tool. The last passage of the tool leaves the 
major burr on the exit side of the machined surface. 
The minor burr is found on the entrance side and 
also on each ridge between tool grooves on the en- 
tire machined surface where they contribute to the 
roughness configuration. 

The burrs developed while the part is in the fixture 
are normally of no concern to the fixture designer 
since they are formed out in the open. This applies 
to machining with a single-point tool and a milling 
cutter (see Fig. 8-2 a and b), but not to drilling (see 
diagram c) as, in this case, the burrs are formed in- 
side the jig and space must be provided where they 
are going to form. The necessary space for the major 


K xi iv v 

F(g. 8-2. a. Major and minor burr formation by a single- 
point tool; b. A milling cutter; c. A twist drill. 

burr is automatically provided for if the part is 
located clear of the fixture base; if not, a hole of 
generous size must be provided in the fixture base 
for the drill to clear and the major burr to form. On 
the top side, clearance is provided between the drill 
bushing and the upper surface of the part for the 
minor burr and for other purposes to be discussed 

Chip Removal 

Chips falling on the fixture present no more of a 
problem than those failing on a part without a fix- 
ture and they are easily scraped or brushed away. 
The most efficient assist in chip disposal is a strong 
flow of coolant, usually in excess of what the cutter 
requires for cooling alone. 

A few minor yet important points must be con- 
sidered by the fixture designer. The means for 
coolant supply and coolant return must be adequate. 
This requires a check on the pump capacity, pipe 
and nozzle dimensions, and the size and extension of 
the collecting grooves and channels in the machine 
tool table and substructure -including strainers, 
sieves, settling tanks, and filters. The flow pattern 
of a stream of coolant discharged to horizontal sur- 
faces is not adequately controlled by the direction 
of the nozzle; it will splash and spray by the impact, 
spread over larger areas and overflow the edges in 
an unpredictable manner. This calls for troughs and 
pans to collect the liquid and baffle plates to protect 
the machine surroundings, including the operator, 
Shields and baffle plates for chips and coolant can be 
made wholly or partly from wire screen or Plexi- 
glas to maintain visibility of the operating area. 
Blowing chips away with an air jet is a double- 
edged sword and is endlessly discussed in the litera- 
ture without a final answer; it is highly efficient in 
moving chips (particularly smaller ones), chip frag- 
ments, and dust; but not in removing them, because 
they settle down in some other place when they are 
outside of the air stream. They contaminate the 
workshop atmosphere with abrasive dust which finds 
its way into slideways and bearings in machines-and 
into the noses, ears, and clothing of the operators. 
The disadvantages of the use of air blast for 
cleaning can be greatly reduced by skillful use of 
shields, baffle plates, chutes, and ducts. An example 
is shown in Fig. 8-3. A mechanical jig with a fixed 
locator plate A and a movable jig plate B is equipped 
with two airjets C and an air valve D, actuated by 
the jig plate in its open position. The air blast 
cleans the jig plate and the locator plate before the 
part E is inserted in the fixture. The chips im- 




Fig. 8-3. Chip removal by an air blast and with a chute. 

pinge upon and are deflected by a baffle plate and 
slide down chute F. The inconveniences of air blast 
are eliminated when suction is used. This is com- 
mon in dry grinding and can be used for cast iron, 
aluminum, and magnesium chips. It is also widely 
used for catching chips produced when machining 
many types of plastics. 

Chips that collect in quantity inside the fixture 
must be given an exit; for this purpose the fixture 
walls are provided with openings (windows) as large 
as the stress analysis can permit, with good clearance 
around the part for manual cleaning access with a 
rake, fork, scraper, or brush. Helpful also is the 
installation, where possible, of a gravity chute as 
chips are easily started down the slope by the vibra- 
tions from the machine. 

Another, and somewhat unusual, example of chip 
removal by a chute involves the drill jig shown in 
Fig, 8-4, The part is a cylinder with two flanges 

Fig. 8-4. Chip removal by a chute built into a drill jig. 

that are to have holes drilled in them. The cylinder 
is bored and the flanges are faced . The part A enters 
the jig through one of the large windows in the side 
walls, and is centered by two plugs B, sliding in 
bores in the upper and lower end wall of the jig. 
With the jig set on end, the part with the plugs in 
position slides down until its lower flange rests on 
the jig. In this position the holes in the upper flange 
are drilled. To drill the holes in the lower flange, 
the jig is reversed. Chips are removed by the chip 
deflector C consisting of a split ring with a V-shaped 
cross section. Only one-half of the chip deflector 
is removed to unload and load the jig. 

Relief and Protection 

The greatest problem, one that confronts the fix- 
ture designer at all phases of his work, is caused by 
chip fragments and dirt; i.e., essentially chip dust 
and particles of rust, scale, paint, and remnants of 
foundry sand from castings. Collecting in corners 
and cavities they cause misalignment by preventing 
proper contact between part and locators. 

Vertical surfaces are, naturally, less exposed to 
dirt accumulations. Horizontal locating surfaces are 
relatively easily cleaned; this is one reason why 
locating surfaces should be kept small and why side 
and end stops preferably should be installed from 
vertical walls. 

When a flat surface with sharp edges slides over 
another flat surface the ieading edge acts as a scraper. 
In this way the edges of locating pads help to clean 
the surface of the part. Occasionally, additional 
grooves with sharp edges are cut into locating pads to 
improve the cleaning effect. In the same way, the 
edge of the part cleans the locator surface, provided 
the dirt has a place to go. This leads to the general 
rule for dirt -proof design: A contact surface shall 
be surrounded by a relief space. Incidentally, 
relief spaces for dirt will also serve as relief spaces 
for burrs from previous operations. They do not 
accommodate large chip accumulations, but they 
allow space for the dirt to escape when it is pushed, 
swept, or scraped ahead of an edge on an entering 

Any horizontal locating surface satisfies the con- 
dition above if it is elevated sufficiently over the 
fixture base. (Examples were seen in Fig. 6-5,) 
Additional constructive action must be taken in the 
corners by providing relief space either in the fixture 
base or in the locator. Pins and buttons can be 
installed in bored holes with a chamfer or a straight 
recess, as in Fig. 8-5a, b, and c. (This solution 
requires an additional and rather expensive machin- 

Ch. 8 



^ I 





!i w 




Fig. 8-5. Locators with relief places for chips and dirt, 

ing operation on the fixture; it is cheaper to provide 
the relief in the locator.) Buttons and pins can be 
machined with a flat recess as in d, and e. The 
latter design has the advantage that it does not re- 
duce the bearing area between the shank and the 
hole. The same applies to the designs in f and g,- 
where a flat locating surface is provided on the pin 
and button. However, the preferred solution is to 
turn a circular recess into the pin or the button as 
in h and i. The additional turning operation to make 
the recess (which does not have to be ground) is 
inexpensive and the bearing area on the shank is not 
significantly reduced. Straight side and end locators 
are made with a chamfered or straight recess as was 
shown in Fig. 6-t2d and e and as in Fig. 8-6. 

In some cases it is possible to let a chamfered 
recess on a side or end stop perform two functions 
as shown in Fig. 8-7. The part is flat and the side 
locator is made higher than the part. The locating 

Fig. 8-7. A side locator with an inclined locating surface. 

surface is inclined to a point above the edge of the 
part. With the clamping pressure applied, as indi- 
cated by the arrow, the part is not only located and 
clamped sideways, but is also forced down on the 
base. The angle of inclination is from 7 to 10 
degrees. This provides sufficient holding pressure 
without risk of jamming or wedging. The same 
arrangement can be used for other types of locators 
(nesting blocks, V-blocks, etc.). It requires fairly 
close tolerances on the height of the part. The 
horizontal line through A does the locating. The 
coordinates x and y are critical dimensions because 
they define the position of A, within the fixture. 
In cases where the two perpendicular base and side 
locating surfaces meet in a corner, a relief groove 
is cut in the corner. The details may vary from case 
to case. Four different configurations are shown in 
Fig. 8-8. 





Fig. 8-6. A side locator with and without relief space. 

Fig. 8-8. Typical relief recesses in coiners. 

The design of dirt relief spaces for circular locators 
follows the same lines as those previously described. 
An example is shown in Fig. 8-9. In this case, as 
well as for pins and buttons, the groove actually 



Ch. 8 



Fig. 8-9. Relief space in a circular locator. 

performs three functions; it provides space for dirt, 
relief for burrs, and clearance for grinding operations. 

Shields and Seals 

A fixture with moving parts, such as bearings and 
sliding pins and wedges, is actually a piece of 
machinery. As such, it requires protection against 
dust getting to its bearing surfaces. Reasonably 
good protection is offered by the use of dust caps, 
as shown in Fig, 8-1 Oa. More effective protection 
is obtained with felt washers (Fig. 8-1 Ob) but they 
have the disadvantage of a limited life; they must 

be kept oiled and renewed from time to time. 
Modern technology offers a variety of seals with 
excellent, although not infinite, service life. The 
O-ring is a well-known example. Where an effective 
dust seal is required, the fixture designer should 
consult catalogs on available sealing components. 

Indexing fixtures represent an important class of 
fixtures with moving parts. The required precision 
can only be maintained if all bearing surfaces are 
wel! protected. The typical design of an indexing 
fixture is shown in Fig. 8-1 1, The center pin (king 
pin) is shielded by means of a cap, A . Seals B are 
provided to protect the index pin and the main 
bearing surface. 

Shielding and sealing, as described, is required 
when a movable component is in an exposed posi- 
tion. Many fixture designs present an automatic 
shielding device at no cost, namely, the part itself. 
A large flat part provides quite an effective shield 
against chips falling on base locators and intermediate 
supports. To fully utilize this effect, locators are 
placed a short distance inside the contour of the 

Fig, 8-10. Chip protection by a shield (a), and a seal (b). Fig. 8-11. Shield and seal applied to an indexing fixture. 




Centering is an advanced method of locating. 
While locating, as previously described, brings one 
surface at a time into the proper place relative to 
the fixture, centering is applied to two surfaces at a 
time and locates a plane within the part— almost al- 
ways the middle plane between the two surfaces. 

Centering has three degrees. Single centering is 
when one middle plane is located, double centering 
is when two middle planes (usually perpendicular) 
are located, full centering is when three middle 
planes (likewise usually perpendicular) are located. 
The components for centering are termed "cen- 
tralizers." Arrows 1,1; 2,2; and 3,3 in Fig. 9-1 indi- 
cate three pairs of centralizers. Single centering 
with one pair of centralizers 1,1 locates the middle 
plane aa and nothing else. Double centering with 
the additional pair of centralizers 2,2 locates two 
middle planes aa and bb, as well as the axis 3,3 
where aa and bb intersect. Full centering with the 
further addition of centralizers 3,3 locates three 
middle planes aa, bb, and cc, three axes 1,1; 2,2; 
and 3,3; and the common center for middle planes 
and axes. 


In the introductory discussion to locating, it has 
been stated that one purpose of a fixture is to ap- 
proximately substitute for the scribed lines and 
punched centers provided in the initial layout of a 
rough part— prior to machining. It was also ex- 
plained that the part locates within the fixture from 
one or several of its surfaces. With tolerances, some- 
times quite wide on a rough part, there is no guaran- 
tee that the physical middle planes, axes, and cen- 
ters of the rough part will coincide with the corres- 
ponding planes, axes, and centers of the fixture. 
The fixture can only guarantee the correct location 

of machined surfaces relative to the locators and to 
each other, regardless of rough part tolerances. The 
application of centering devices has advanced the 
function of the fixture to the point where it pro- 
vides a true substitute for the scribed and punched 
layout markings. The physical middle planes, axes, 
and centers within the part are now exactly located 
in the fixture (within tolerances). Machining allow- 
ances are evenly distributed, depth of cut is constant 
on all sides, and excessive cutting forces are avoided. 
Center of gravity is also correctly located and any 
unbalance of rotating parts (in turning operations) is 
eliminated. Surfaces that remain unmachmed are 
more accurately located relative to system lines and 
planes in the part and in the completed product. 
Entire machining operations may be eliminated by 
the use of cold rolled and cold drawn stock located 
and clamped in accurately centering devices. 

Centralizers and Locators 

Centralizers are single or multiple components. 
They act as locators, as clamps, or both, A fixed 
single component centralizer is a locator. Multiple 
component centralizers have at least one movable 
part. They can have one fixed (a locator), and one 
or more movable components (clamps). When all 
components of a centralizer are movable, they can 
be considered as either clamps or locators. Then 
there is no real distinction between locators and 

Typical combinations of locators and centralizers 
are shown schematically in Fig, 9-2. Double center- 
ing is accomplished whenever an axis is located, re- 
gardless of what system of centralizers is used. By 
this definition the common three-jaw, self-centering 
chuck and the collet chuck are double-centering de- 
vices (see diagram g), as is the two-jaw chuck fre- 
quently used on smaller turret lathes (see diagram e). 
Drill chucks are also double-centering devices. The 




Ch. 9 

Fig. 9-1 . Single, double, and full centering. 

machine tool vise with two V-grooves (see diagram 
f), is a single-centering device. Centralizers are not 
nesting components. They provide positive contact 

and pressure without clearance. Locating with cen- 
tralizers is, therefore, more closely defined and more 
accurate than nesting, particularly on rough parts. 

Ch. 9 





////,, /s 




l"! -i 


Fig. 9-2. Typical combinations of centralizers and locators: a. Single defined, not centered; b. Double defined, not 
centered; c. Single centered; d. Single centered; e. Double centered; f. Single centered; g. Double centered; 
h. Single centered; i. Single centered. 

Centralizers can be classified into three categories: 

1 . The angular block type 

2. The linkage controlled multiple centralizer 

(automatic or scissor-type) 
3. Self-centering chucks of commercial types. 

The term "angular block" is the common designa- 
tion for block-type locators with converging or di- 
verging locating surfaces. They are used in three 



Ch. 9 

forms, the V-bloek, cone locator, and spherical 
locator. The V-block has two flat surfaces. In the 
most widely used type of V-block the V is concave; 
the two surfaces form a slot or groove which receives 
the part. An inverted configuration is used occasion- 
ally where the V is convex and forms the two sur- 
faces of a triangular prism which centralizes by en- 
tering the space between the prongs of a fork-shaped 
part. The cone locator has a convex (male) or con- 
cave (cup, female) conical surface. The included 
angle within the locating surfaces is significant for 
the function of the locator. The locating surface of 
the spherical locator is formed as a spherical cap or 
ring and can be convex or concave. 

Linkage controlled multiple centralizers are those 
in which the movable components are so connected 
that they maintain equal distance from the middle 
plane, the axis, or the center. One example is a pair 
of scissors, and scissor-like linkages are frequently, 
but not exclusively, used. The term "linkage" is 
here used in a wider sense; it includes mechanisms 
that act with cams, wedges, telescoping rods, sym- 
metrically arranged springs, etc. In mechanical lan- 
guage these are all called kinematic chains. 

Centralizers and Equalizers 

In a description of fixture design there are two 
terms that must not be confused: "centralizers" 
and "equalizers." Both use linkages, but they serve 
entirely different, almost opposite, purposes. In 
centralizers, the linkage system controls the motion 
and position of the locating and clamping points, 
and forces the part into a position defined by these 
points. Equalizers are also linkages and are used for 
clamps for the purpose of equalizing the clamping 
forces on uneven surfaces of the part. The equali- 
zers carry the clamping points and enable the in- 
dividual points to adjust themselves back and forth 
to accommodate local irregularities. They do not 
force the part into a position; they are forced into 
position by the part. 

Commercial Centralizers, Chucks 

Self-centering lathe chucks have jaws which are 
made to move in a concentric relationship to the 
spindle axis. They can therefore be classified as 
centralizers and are available with two or three jaws. 
The four-jaw chuck is also a device for centering, 
but the jaws are adjusted individually and manually. 
All these devices are commercially available, general- 
purpose work holders; as such, they do not qualify 
as fixtures. However, they become fixtures, or rather 

fixture bases or fixture bodies, when they are pro- 
vided with special jaws or jaw inserts and locators to 
fit specific parts. Machine tool vises are also com- 
mercially available, general-purpose work holders, 
not fixtures. They are not, in themselves, centrali- 
zers, but they too can be fitted with special inserts 
and other components for special work. 

A general-purpose work holder, "borrowed" from 
its parent machine and mounted on a base, can serve 
as a major fixture component with other compon- 
ents built upon it or around it. Drill chucks are used 
to good advantage for holding small parts. Instead 
of physically acquiring a piece of general-purpose 
equipment, the fixture designer may feel inspired by 
its design principle and apply it to the assignment in 
hand. The principle of the collet chuck with one or 
two sets of contracting or expanding elastic fingers 
can be applied to centralizers that can be mounted 
on an arbor or on a base. Fixtures of this type are 
excellent for locating and clamping on internal and 
external cylindrical or conical machined surfaces 
where high precision is required and the load from 
the machining operation is relatively light. The par- 
ticular advantage is that they exert a uniform pres- 
sure on the part and do not force it out-of-round. 
They do have one serious limitation, however, as 
shown in Fig. 9-3; the fingers bend and their slope 
varies as they expand or contract. Theoretically, 
there is only one position where they grip and locate 
with their full surface. In actual practice this means 

Fig. 9-3. The principle of the collet chuck. 




that they operate satisfactorily only over a small di- 
ameter range; inside and outside of that range they 
grip only with the edge. 

The inclined angle on the conical surface of the 
centralizer is 30 degrees; the activating (mating) 
cone is made with 3 1 degrees if it acts on the inside 
of the fingers as shown in Fig. 9-3, and 29 degrees if 
it acts on the outside. The thickness of the fingers 
must be small to ensure sufficient flexibility. For 
work diameters D, ranging from 1/4 inch to 6 inches 
(6 to 150 mm), the recommended thickness t varies 
from 5/64 inch to 13/64 inch (2 to 5 mm). 

The question may arise whether hollow parts 
should be located (centered) on the inside or on the 
outside; if so, the following points may be con- 
sidered: Any radial locating error (eccentricity) is 
reproduced to true size, not more, not less, regard- 
less of -where the part is located. A misalignment 
error (a wobble) is reproduced to true size if the 
part is centered on the outside, but is reproduced 
with a magnification if the part is centered on the 
inside and on a small diameter- the magnification of 
the error increases with the diameter ratio. 

For a given clamping pressure, the transmitted 
torque and the maximum permissible size of cut is 
greater when the part is clamped on the outside; 
also, with outside clamping the part is less likely to 
slip in case of an accidental overload in the cut. 

Centering by Means of V-Blocks 

The common method of centering cylindrical 
pieces or surfaces in a V-block is shown in Fig. 9-4. 
The V-block, as a rule, is stationary, held in place by 
screws and dowel pins, as indicated in the figure. 
However, the V-block may also be adjustable in 
order to take up the variations of the pieces placed 
in it, and in order to act as a clamp. A V-block of 
this type is shown in Fig. 9-5. Here, A is the adjust- 
able V-block, having an oblong hole B, to allow for 
adjustment. The block is held in place by a collar- 
head screw C, which passes through the elongated 


rj-r j 


hole. The underside of the block is provided with a 
tongue £>, which enters into a slot in the jig body 
itself, the V-block thereby being prevented from 
turning sideways. The screw E, passes through the 
wall of the jig, or through some lug, and prevents 
the V-block from sliding back when the work is in- 
serted into the jig. It is also used for adjusting the 
V-block and, in some cases, for clamping the work. 
V-blocks are usually made of machine steel, but 
when larger sizes are needed they may be made of 
cast iron. Little is gained, however, in using cast 
iron, as most of the surfaces have to be machined, 
and the difference in the cost of material on such a 
comparatively small piece is very slight. 

Fig. 9-4. Centering a cylindrical surface by means of a 

I'ig, 9-5. An adjustable V-block used as a locator and 

For large size V-blocks it is economical to use 
finish machined, cast iron V-block stock, commer- 
cially available in widths up to 4 1/2 inches (120 mm) 
and in lengths from 2 to 3 feet (600 mm to 1 m). 
When a V-block is used for locating round parts, 
there is so much empty space left that there is no 
particular need for a relief groove for catching chips 
and dirt. When a relief groove is used, as it often is, 
the purpose is to provide clearance for the grinding 
wheel used for finishing the flats of the V-block. 
The groove must be made with rounded corners or 
as a semicircle, to reduce stress concentration. 

Much mathematics has been applied in attempts 
to find and to justify an optimum value for the in- 
cluded angle, but no convincing calculation has yet 
appeared in the literature. Some extreme limits can 
be easily established. A V-block with a 30-degree 
angle will hold a part very firmly and is near the 
point where the part becomes wedged by friction. 
A small diameter variation causes a large variation 
in the height at which the part rests in the V. Thirty 
degrees is clearly a lower limit, and not even a prac- 
tical one. A V-block with a 120-degree angle will 
receive the part freely and is not very sensitive to 
diameter variations. The position of the part is not 
very stable; it takes a relatively small horizontal 
force at the clamping point to roll the part out of 




its resting place. 120 degrees is dearly an upper 
limit and not a desirable one. 

Industry has solved the problem by accepting, al- 
most universally, the value of 90 degrees for the 
included angle. Most V-block components in com- 
mercial fixtures are made with this angle, and V- 
hlock stock (rough and machined castings) is com- 
mercially available with tolerances down to 90 de- 
grees ±10' for the included angle and ±0.002 inch 
per foot (0.2 mm per m) for straightness. 

The 90-degree V-block is popular and rightly so. 
It provides a good stable support for circular cylin- 
drical parts and is insensitive to even grossly inac- 
curate application of the clamping force (Fig. 9-6). 
With the clamping force acting on the top of the 
cylinder, it can deviate ±22 1/2 degrees from the ver- 
tical direction before the position of the part be- 
comes unstable and it starts rolling. The point of 
action of the clamping force can move 45 degrees to 
either side before stability is lost. Other advantages 
of the V-block are that it is solid, strong, and rigid; 
it provides good bearing areas, is suitable for long as 
well as for large parts, lends additional stability and 
strength to the fixture, is versatile in its applications, 
and is inexpensive. 

Fig. 9-6. The stability range of the 90-degiee V-bloek. 

machining of any surface and configuration that is 
symmetrical with respect to the bisector plane, or 
which is dimensioned entirely and solely relative to 
this plane. Examples of such configurations are (see 
Fig. 9-7) holes and slots passing through or across 
the part in the plane of symmetry, and planes paral- 
lel to that plane. The words "through or across" are 
significant. Consider a blind hole or longitudinal 
key seats (see diagram e). They are machined in per- 
fect symmetry, but to a depth that depends on the 
physical diameter of the part. In a great many cases 
this objection is academic only, as diameter varia- 
tions within tolerances are small, and blind holes, 
keyseats, and similar configurations are usually de- 
signed with a generous depth tolerance that can ab- 
sorb the small error from the diameter tolerance. 
More serious, perhaps, is the effect of diameter 
variations on the location of configurations that are 
dimensioned relative to the diameter; that is, per- 
pendicular to the bisector (see diagram f), A sym- 
metrically designed hole, keyseat, or slot will move 
clearly out of symmetry with a diameter variation. 
This effect is so obvious that many sources call it a 
wrong use of the V-block. The criticism is correct 
in principle, but exaggerated in reality; the real prob- 
lem is again a problem of tolerances (see Fig. 9-8a 
and b). In a, the V-block is used as a centering de- 
vice for a cylindrical part. When A is the variation 
of the part diameter, then the center of the part is 
located on the bisector with a locating error e, de- 
termined by 

e = yAV2 = 0.707A 

In b, the V-block is used as a base and side locator; a 
use for which il is eminently suited. In this applica- 
tion the error in the horizontal (or vertical) direc- 
tion is clearly 

1 . 

Limitations of the V-Block 

Since the V-block has so many easily recognizable 
good points, there is also the danger that it may be 
used for the wrong purposes. Its capability as a cen- 
tralizer is quite limited; taken individually it pro- 
vides only single centering, but it does that well. 
The plane in which it centers the part is the bisector 
of the angle. This centralizing effect is independent 
of the diameter of the part, up to the limit of ca- 
pacity of the V-block, and is used in locating for the 

The Sliding V-Biock 

With the few reservations stated, the V-block with 
a single clamp is very suitable for locating and single- 
centering circular and cylindrical parts. Long parts 
of ample stiffness require a V-block at each end 
rather than one long V-block. The area of applica- 
tion of the V-block principle is significantly ex- 
panded by combining one fixed and one movable 
V-block, the movable V-block acting also as the 
clamp. This system is widely used for elongated 





e f 

fig. 9-7. Application of the V-block to cylindrical parts with different machining configurati 

parts with rounded (partly circular) ends. A typical 
example is shown in Fig. 9-9. 

The drill jig shown is designed for drilling fork 
links. The form of the links is indicated by dot-and- 
dash lines in both views. The link has a round boss 
at one end and rounded forks at the other. It is ac- 
curately held between two V-blocks, one adjustable 
and the other stationary. The adjustable V-block A 
is clamped against the work by a star-wheel and 
screw, and it travels between finished ways, thus 
providing an accurate as weli as a rapid method of 
clamping. These V-blocks have inserted steel plates 
B and C. The latter, which is in the stationary V- 
block, carries a drill bushing for drilling the lower 
fork, and an upper shoulder on this plate provides a 
support for the upper fork; thus there are two bush- 
ings in alignment for drilling the two ends. The in- 
serted plate B in the adjustable block supports the 
opposite end of the fork link. With this arrange- 
ment, a double V-elamping jig is obtained having a 

three-point support. This drill jig was accurate, 
rapid, and easily operated. 

The principle of the sliding V-block can be applied 
to parts of the most diversified shapes, as long as 
they present bosses ot other contours with at least a 
little more than 90 degrees of a circle. 

V-blocks are not often used for locating square 
and otherwise prismatic parts. One reason is that 
such use of the V-block is actually nesting, with its 
inherent lack of accuracy, mainly in the matching of 
the angles. Parts with flat surfaces are better lo- 
cated on base points with side and end stops, on 
strips, or in a vise-type of fixture. Another reason is 
that the V-block provides centering with respect to 
a diagonal, a feature rarely called for. 

Conical Locators 

The conical locator is well known in the machine 
shop, although not necessarily by that name. To 




Fig. 9-8, The effect of diameter tolerance on the locating 

center drill a part prior to a turning operation and 
set the part up on the lathe centers is actually lo- 
cating with conical locators. The tapers in and on 
machine tool spindles and the corresponding tapered 
shanks on drills, arbors, chucks, etc., are conical lo- 
cators, but these tools are not fixtures. The lathe 
mandrel is a work holder with a tapered seat for the 
work. The taper is very small, 0.006 inch per foot 
(0.5 mm per m), A common characteristic of these 
devices is that they transmit torque by friction. The 
lathe mandrel does not define the axial position of 
the part; it is actually determined by the bore toler- 
ance and the amount of pressure used when the part 
is mounted. The lathe mandrel is a general-purpose 
work holder, not a fixture. 

From this brief resume it follows that conical lo- 
cators cannot duplicate these devices. The conical 
locator can center and does that well. It can pro- 
vide some degree of axial locating but not with pre- 
cision. Integrating a conical centralizer and a flat 
axial locator (see Fig. 9-1 0) in one piece requires ex- 
tremely close tolerances and is impractical except 
for special, high-precision work. A good workable 
solution is to mount a sliding conical locator within 
a flat axial locator and provide independent clamp- 
ing devices. The example shown in Fig. 9-1 1 is typi- 
cal of the application of this principle and contains 
some additional features necessitated by the need 
for gripping and clamping the work by a thin rim, 
without distorting it. 

The work A is a special clutch flywheel which has 
been partially machined, Jn order to obtain con- 
centricity of the various surfaces, it is necessary to 
locate the work from the taper in the hub. In order 

Fig. 9-9. Locating and single centering by means of a fixed and a sliding V-block. 

Ch. 9 



Fig. 9-10. Combinations of a conical centraiizer and a flat 
axial locator, a. Proper locating is impossible; 
b and c. Locating is possible, but not useful; 
d. Locating good. 

to compensate for slight variations between the taper 
and other finished surfaces, a tapered, shell-locating 
bushing B is centrally located on the stud C, which 
is held in place in the faceplate fixture E by the nut 
and washer at D . A light coil spring M insures a per- 
fect contact with the tapered surfaces, while a small 
pin N restrains the movement. As the outside of 
the work is to be finished during this setting, it is 
necessary to grip the casting in such a way that the 
clamps will neither interfere with the cutting tools, 
nor cause distortion in the piece itself. With this 
end in view, the three lugs around the rim of the 
fixture are provided with shell bushings K, each of 
which is squared up at its inner end to form a jaw 
which is bored to a radius corresponding with the 
rim of the casting L. It is splined to receive a dog 
screw J, which prevents it from turning, and it also 
gets a good bearing directly under the point where 
the work is held so that there is no danger of it 
springing out of shape. 

The bolts F pass through the shell bushings and 
are furnished with nuts G at their outer ends, the 
nuts having a knurled portion 0, which permits of 
rapid finger adjustment before the final tightening 
with a wrench. It will be seen that this construction 
automatically obtains a metal-to-metal contact with 
the thin flange of the casting, without distorting it 
in the least, as the floating action of the bushings 
equalizes all variations and yet holds the work very 
firmly. After the clamps have been set up tightly, 
they are locked in position by the set-screw H, at 
the rear of the fixture. This application of the float- 
ing principle may be adapted to many kinds of work, 
and the results obtained leave nothing to be desired. 
The machine for which this device was designed is a 
turret lathe of the horizontal type. 

Conical locators do not necessarily require tapered 
bores to work with; they also work very well with 

circular edges. Any part with a cylindrical outer 
surface or a cylindrical bore and one or two flat end 
surfaces can be centered by means of conical loca- 
tors provided that the edge is really circular. This 
requires that the end surface be perpendicular to the 
axis. If the part has been machined, the edges must 
be inspected and any machining burr removed. 
Castings with cored holes are likely to have fins 
around the core prints that must be cleaned away 
before the edge of the hole can be used for locating. 
Whenever it is essential that a cylindrical part of 
the work be located centrally either with the outside 
of a cylindrical surface or with the center of a hole 
passing through the work, good conical locators can 
be designed as shown in Figs. 9-12 and 9-13. In 
Fig. 9-12 the stud, A, is countersunk conically (cup 
locator) to receive the work. The stud is made from 
machine or tool steel, and may, in many cases, serve 
as a bushing for guiding the tool. In Fig. 9-13 the 
stud is turned conically in order to enter a hole in 
the work. These two cone locators are stationary; 
they are only used for locating the work and would 
require additional means for clamping. 

Clamping with a Moving Cone 

Clamping devices for use with cone locators can 
be separate and independent, but it is also possible, 
and very convenient, to make one of two locators 
movable and use it for clamping. The bearing area 
on the edge is small and the clamping load must be 
kept light to avoid deformation of the edge, or other 
damage. Clamping by means of a movable cone lo- 
cator is widely used in connection with drill bush- 
ings. Drill bushings with an external screw thread 
are known as "screw" bushings and may be used for 
locating and clamping purposes by making them 
long enough to project through the walls of the jig 
and by turning a conical point on them, as shown in 
Fig. 9-14, or by countersinking them, as in Fig. 
9-15. In all cases where long guide bushings are 
used, the hole in the bushing ought to be counter- 
bored or recessed for a certain distance of its length. 

In some instances the screw bushing must be mov- 
able sideways, i.e., when the piece of work to be 
made is located by some finished surfaces, and a 
cylindrical part is to be provided with a hole drilled 
exactly in the center of a lug or projection, the rela- 
tion of this hole to the finished surfaces used for lo- 
cating is immaterial. The piece of work, being a 
casting, would naturally be liable to variations be- 
tween the finished surfaces and the center of the 
lug, particularly if there are other surfaces and lugs 
to which the already finished surfaces must corres- 









Fig. 9-12 (Left). The outside conical locator. 
Fig, 9-13 (Right). The inside conical locator. 

pond. In such a case, the fixed bushing for drilling a 
hole that ought to come in the center of the lug, 
might not always suit the casting and so-called 
"floating" bushings, as shown in Fig. 9-16, are used. 
The screw bushing A is conically recessed and lo- 
cates from the projection on the casting. It is fitted 
into another cylindrical piece B, provided with a 
flange on one side. The piece B, again, sets into 
hole C which is large enough to permit the necessary 

Ch. 9 



Fig. 9-14 (Left). Threaded drill bushing used as an inside 

conical locator. 
Fig. 9-15 (Right), A threaded drill bushing used as an 
outside conical locator. 

adjustment of the jig bushing. When the bushing 
has been located concentric with lug E on the work; 
the nut F having a washer G under it, is tightened. 
The flange on piece B and washer G must be large 
enough to cover hole C, even if B is brought over 
against the side of the hole. It is seldom necessary, 
however, to use this floating bushing, for a drilled 
hole in a piece of work rarely can be put in without 
having any direct relation to other holes or surfaces. 

Fig. 9-16. A floating drill bushing used as an outside 
conical locator. 

Linkage Controlled (Automatic) Centralizers 

For the control of the moving components the 
following mechanisms (kinematic chains) are used: 


Actuated by 

Sliding wedges 

Screw and nut 

Screw with right- and left-hand 

thread (turnbuckle principle) 

Opposed springs 

Linkage systems 

Rotating arms and cams 

Linkage systems 

Other cams, including inclined 

flat surfaces 

Symmetrically moving 

Scissor- type linkages 

pairs of levers 

Pantograph systems 

Point or strip locators 

Scissor-type linkages 

Pantograph systems 

The list is representative, but not comprehensive; 
a complete list would require a kinematics encyclo- 
pedia. There is still room for the inventiveness and 
ingenuity of the fixture designer. As a guide, a few 
fundamental rules are given: 

1 . Make it simple 

2. Prefer rotation to sliding 

3. For rotating members— apply forces perpen- 
dicular to radii 

4. For sliding members-provide support below 
the points of force application 

5. Specifically for lever arms in linkages— 

a. Make arms of equal length; if not feasible, 
then operate on the long arm and let the 
short arm perform the clamping 

b. Should this not be feasible, operate through 
a force -magnifying device, such as a screw 
mechanism with a handwheel 

6. For all mechanisms— 

a. Watch for rigidity of all individual members, 
and for 

b. backlash in all bearings and other points of 

To illustrate these rules, a few typical examples 

Kinematic chains do not squeeze and clamp as 
hard as single clamps, because the available force is 
divided between several locating and clamping points. 
Primarily used for drill jigs rather than for milling 
fixtures, they are recommended when drilling flat 
plates and covers which are not usually machined at 
the sides but have to be gripped or located in a jig 
by their rough cast edges. Similarly, such self-cen- 
tralizing features will be found advantageous, time- 
saving, and economical in drilling parts having a simi- 
lar shape, but whose overall dimensions differ. 

One of the simplest forms of a self -centralizing de- 
vice for a drill jig is shown in Fig. 9-17. This jig is 
an example of the type with sliding wedges actuated 
by opposing springs. It can also be characterized as 
a jig with a split V-block and contains a rectangular 
cast-iron body A, which is flanged at the bottom for 
hold-down purposes. A swinging arm B is pivoted 
on a pin C that is pressed into both side walls of a 
slotted boss on top of the body. Arm B carries the 
drill bushing D. Pressed into the underside of the 
arm on each side of the bushing, are two bearing 
pads E. When the arm is in the horizontal position, 
as shown, these pads will press equally on the work- 
piece X, holding it firmly to the top of the jig body. 
The right-hand end of arm B has an open-end slot 
for the cylindrical shank of clamping stud F. A 



Ch. 9 



SO £■ O X C 


Fig. 9-17. A centralizing drill jig with spring-actuated wedges. 

knurled nut G, threaded on the upper end of this 
stud, enables the arm to be clamped to the work. 
The stud can be swung about a pivot pin H pressed 
into the body. The centralizing action on the work- 
piece is obtained from identically shaped spring- 
loaded slides J, which are mounted in a guide hole 
K, drilled completely through the body. Springs L 
are held in pockets in these slides by stop-plates M , 
which are fastened to the sides of the jig body by 
screws. Each slide has a vertical projection at the 
inner end, with the hardened end faces of these 
projections inclined at an angle of approximately 10 
degrees. The projections slide in slots which extend 
from the top edge of the jig body into the bearing 
holes K. The springs force the slides inward, toward 
each other, and the extent of this movement is lim- 
ited by either the end faces of the slots or the work- 
piece, as shown. 

A large-diameter vertical hole extends down 
through the body, directly below the hole to be 
drilled in the workpiece, to permit chips to fall out 
of the jig. A short cylindrical plug, cross-drilled as 
shown, is tightly pressed into hole K to lie midway 
between slides /. This plug prevents chips from en- 
tering hole K. To mount a workpiece in the jig, arm 
B is swung upward. The two opposing sides / will 
be in their innermost positions, in contact with the 
end faces of the slots, and the workpiece is placed 
between the inclined faces of the slides. Transverse 
location of the workplace is obtained by butting it 
against a plate O fastened to the top face of the jig 
body. Arm B is then lowered into the horizontal 
position and clamping stud F is swung into its verti- 
cal position, as shown. As nut G is tightened, pads 
E bear down on the workpiece and press it into con- 
tact with the top of the jig body. This action will 
cause the slides to move apart an equal amount, and 
the workpiece X will become located centrally with 
relation to bushing D. Parts differing considerably 
in width can be accommodated without difficulty in 

such a jig, since the slides can be arranged with an 
appreciable amount of opening and closing. Also, 
the jig may easily be adapted to increase these 
facilities merely by altering the angle of taper on 
the slides. 

A typical design of a fully linkage-operated cen- 
tralizer is shown in Fig. 9-1 8. It differs from the one 
shown in Fig. 9-17 by two very important features. 
The first is that the centralizing motion is positively 
controlled by the linkage, while in the previous case 
it was dependent on the symmetry in the spring ar- 
rangement. The second feature is that the effective 
opening has a large operating range so that this fix- 
ture can be successfully employed in drilling parts 
having considerable variation in width or length. As 
with the jig previously described, a swinging arm B 
is pivoted on a pin C, which is pressed into jig body 
A . A drill bushing D is carried in the arm, and lo- 
cated on each side of this bushing are the identical 
bearing pads E. The right-hand end of arm B is 
slotted to admit clamping stud F, which is fitted 
with a knurled nut G and pivoted on pin//. 

The bellcrank locating and clamping levers/ area 
sliding fit within a narrow slot in the jig body, and 
pivoted on pins K pressed into the body. Perma- 
nently fitted into a transverse slot in the body is a 
platform L for supporting the workpiece X. Vertical 
clearance holes are provided in this platform, and in 
the jig body, to permit the chips to fall through. 

The upper inner edges of levers J , which contact 
the sides of the workpiece, are rounded and har- 
dened, and can be serrated to provide a better grip. 
The lower ends of the levers are reduced to half 
their total thickness so that they overlap, and the 
left-hand lever is slotted to fit over pin M pressed 
into the tail of the right-hand lever. When the lower 
halves of the levers are in a horizontal position, the 
center of pin M is aligned with the vertical center- 
lines of the jig body and drill bushing. This arrange- 
ment insures that the levers will be swiveled equally. 




Fig. 9-1 8. A fully linkage-operated centralizei. 

Actuation of the levers is obtained by means of rod 
N, the slotted shackle end of which is pinned to the 
right-hand lever. The cylindrical shank of rod A r is a 
running fit within an externally threaded sleeve O, 
which is screwed into the right-hand wall of body A* 
When hand wheel P is rotated, levers J will be swiv- 
eled (due to the force of sleeve 0} against the shoul- 
der on rod N or the collar pinned to the rod. 

The manner of loading and using this jig is similar 
to the one previously described. With arm B raised, 
workpiece X is placed on platform L. The contact 
surfaces of levers J will have been moved apart by 
rotating handwheel P. Transverse location of the 
workpiece is obtained by butting it against an ad- 
justably mounted stop-plate Q, The handwheel is 
then rotated in the opposite direction until the 
work is firmly gripped and centralized by the con- 
tact surfaces of the levers. Arm B is then returned 
to the horizontal position shown, and clamped by 

tightening nut G. Additional clamping pressure is 
thus exerted on the work by pads E. 

An example of the use of rotating arms actuated 
by inclined flats is shown in Fig. 9-19. This type of 
self-centralizing jig has been proved economical and 
accurate in drilling uniformly central holes through 
thin cover plates and similar parts having large varia- 
tions in width. In this jig, a swinging arm B is again 
pivoted on a pin C, which is pressed into the jig 
body A. The arm carries a drill bushing D, and its 
right hand is slotted for a ring-head clamping bolt E 
that carries nut F and is pivoted on pin G. 

A spring-loaded cylindrical plug H is a sliding fit in 
the vertical bore of the jig body. Rotation of the 
plug is prevented by key N. The lower threaded end 
of the plug is screwed through handwheel K which 
is carried in a horizontal slot in the jig body. The 
upper head of the plug, on which the workpiece X 
rests, has two diametrically opposite slots into which 



Ch. 9 

Fig. 9-19, A cam -operated centraliier. 

are fitted the triangular-shaped locator pads/. These 
pads pivot on pins L, pressed into the side walls of 
the slots. The lower, outer corner of each locator 
pad rests on the tapered bottom surfaces of slots cut 

in the side walls of the jig body. In operation, arm 
B is raised, and a work piece is placed on top of plug 
H, located against a fixed pin or plate, not shown. 
Then, by turning handwheel K, plug H is drawn 
downward— against the action of spring M— and pads 
/ are pivoted toward each other, thus centralizing 
and gripping the workpiece. Arm B is then lowered 
into a horizontal position and clamped by tighten- 
ing nut F. 

Wear can be minimized in this jig by screwing 
hardened head set-screws into the tapered bottom 
surfaces of the slots cut in the side walls of the jig 
body. The rounded contact points of pads/ would 
then bear on the heads, and slight adjustments could 
be made by tightening or loosening the set-screws. 

An effective method of obtaining centralization of 
the work by means of a real linkage (in this case, a 
linkage of the pantograph type) is illustrated in the 
drill jig seen in Fig. 9-20. Secured to the top, near 
the rear edge of jig body A , is a slotted bracket B. 
A swinging arm C, pivoted about a pin pressed into 
the uprights of bracket B, carries drill bushing D. 
The forward end of this arm is slotted to admit the 
shank of a ring-head clamping bolt E that carries a 
knurled clamping nutF. The ring head of the clamp- 
ing bolt is a close fit in a slot on bracket G secured 

Fig. 9-20. A linkage-operated centralizer with a pantograph mechanism. 

Ch. 9 



to the front of the body and pivots about a pin 
pressed into the uprights of this bracket. 

Workpiece X is placed on the top surface of jig 
body A, bearing against stop-plate H secured to the 
body for lengthwise location. The part is centralized 
transversely and gripped by means of two bars /, 
which rest on the smooth top surface of the jig body. 
The inner contacting surface of these bars are re- 
lieved slightly to reduce the frictional pressure on 
the workpiece. The ends of both bars are pinned to 
levers K, forming a pantograph mechanism. Levers 
K can be pivoted about studs L. The front end of 
the extended right-hand lever K has an elongated 
slot to fit around a pin M, pressed into the slotted 
end of rod N. The threaded shank of rod N passes 
through plain holes in both walls of a slotted bracket 
O secured to the right-hand edge of the jig body, 
and is screwed in the internally threaded handwheel 
P. Thus, when the handwheel is rotated, the panto- 
graph lever system is swiveled about studs L, either 
moving bars J together to clamp the work, or apart 
to permit loading and unloading. Arm C is handled 
in the same manner as those on the jigs previously 
described; swung upward while unloading and re- 
loading, and down into the position shown, when 
the workpiece is in place. 

Centralizes for Gear Wheels 

circle. They are the machining of a localized detail 
such as the drilling of a hole pattern or the milling 
or broaching of a key seat, and the finishing of the 
bore concentric with the actual pitch circle as de- 
fined collectively by the teeth, 

A localized detail is usually dimensioned from a 
tooth or tooth space and must be located according- 
ly. This is not just a case of radial locating from one 
side only, but the locator must pick up both sides of 
the tooth or tooth space and locate with respect to 
the bisector between them . The locating is a cen- 
tralizing operation and can be done with a sliding V- 
block on a tooth or a sliding V-prism in a tooth 
space (see Fig. 9-21). The design of these com- 
ponents is based on the gear tooth geometry, as de- 
fined by the angles <p, b } , and b 2 . 

To ensure contact on the pitch circle, the included 
angle a in the vee is determined by 

for a V-block 

= T + *, 
fl = 2(0-*i) 

for a V-prism 

In the manufacture of gear wheels there are two 
types of operations which require locating of the 
gear from points on the tooth flanks in the pitch 

a = 2(<j> + b 2 ) 

Fig, 9-21. Centralizers for gear wheels. 



Ch. 9 

<t> is the pressure angle; the angles b\ and b 2 de- 
pend on the gear data and the tolerances (backlash). 
All angles are in degrees. Using symbols in Machin- 
ery 's Handbook, 1 9th edition, the gear data are: 

Diametral pitch 


Number of teeth 

N = Pl 

Pitch diameter 

D = j 

Chordal thickness 


Tooth space chordal width 


Backlash (circular) 


t = D sin b\ s = D s\nb 2 

Then, disregarding backlash 

h -h - 90 ° 

and with backlash 

b, + b 2 =■ 


*a =*!+(§ X57.3°) 

Suggested values for backlash: 

... _ 0.030 , 0.040 

Minimum B ■ » , Average £ = — = — , 

Maximum B — 


In connection with the metric system, gear di- 
mensions are based on the module which is denoted 
m, and is always expressed in mm. Modules are 
standardized in simple and round numbers. The re- 
lations between pitch and module are: 

l= 2SA(J_\ 

m Vinch/ 


and the suggested backlash values, now expressed 
in mm, are: 

Minimum B = 0,030 m, Average B = 0,040 m, 
Maximum B = 0.050 m 

For centralizing in a tooth space, the V-prism can 
be replaced by a cone of the same included angle 
and by a circular pin or spherical ball, of radius R 

R = 

2 cos ■ 

2 cos 

+ M 

For centering a gear wheel from its pitch circle, a 
plurality (usually three, but sometimes four) of cen- 
tra lizers are used evenly spaced andmountedonthe 
jaws of a self-centering chuck, If the centralizers are 
circular pins, the capacity of the chuck must be at 
least D Q , determined by 

D Q = D cos b 2 + 2R (\ + sin j\ 

n t. j, /l + sin(0 + 6 2 )\ 

= I) CDS b-, + s\ 7 , , , — r^M 

= Dcosb 2 + s tan 1/2(90° + + b 2 ) 

Cones and balls are also used as centralizers for 
holding helical gears. Straight solid pins cannot be 
used as they are not compatible with the helix (ex- 
cept when very short), but pins formed as cylindri- 
cal spring rolls and held in position so as to form a 
cage, are used for locating and holding helical gears 
between chuck jaws. Figure 9-22 shows four such 
pins in position. 

The really "natural" centra lizer for a gear is an- 
other gear or a section of a gear. A fixture for bevel 

Courtesy of Heald Machine Div. , Cincinnati Milacron Inc. 
Fig. 9-22. A centralizing fixture for a helical gear, a. Cylin- 
drical spring rolls for gripping the gear tooth 
spaces; b. The complete grinding fixture. 

Ch. 9 



gears is shown in Fig. 9-23. ' Each centraiizer is an 
insert with one gear tooth operating in a tooth 
space. The inserts can slide radially and are forced 
inward by the outer cone when the base plate is 
pulled lengthwise toward the spindle. The fixture 
shown in Fig. 9-24 has a cluster of small pinions 

Example- A spur gear with 20 degree pressure angle 
has 6 diametral pitch and 72 teeth. A V-block loca- 
tor shall be designed with and without considera- 
tions of average backlash. 

D =• 

1 2 inches 

Without backlash 

ft -2£- 

61 "72 - 

3 = 2(20' 
With backlash 


-1°15') = 37°30' 

B = 

= 0.0067 inch 

Courtesy of E, Thaulow 
Fig. 9-23. A centralizing fixture for a bevel gear. 

bi=^p: rrrvrX 57.3 =1 15 —58 

= ri4'02" 
a = 2 (20° - 1°14'02") = 37°3l'56" 

Courtesy of Heaid Machine Div., Cincinnati Mitacron Inc. 
Fig. 9-24. Centralizing fixtures for clamping helical gear workpieces by means of a cluster of small pinions. (The Garrison 
gear chuck.) a. The fixture open, helical gear workpiece in foreground; b. The fixture with helical gear work- 
piece in place. 

acting as centralizers for external or in.ernal spur 

and helical gears. Kadi pinion is mounted on an ec- 
centric pivot and the work is clamped ar.d centered 
relative to its pitch circle when all pinions are simul- 
taneously rotated. 

E, Thaulow, Maskinarbejde 
Gad's Forlag, 1930) vol. II. 

(Copenhagen: G.E.C. 

Using symbols in Machinery's Handbook, 19th 
edition, a helical gear is characterized by the helix 
(spiral) angle a, also known as the "tooth" angle, 
which is the acute angle between a tooth and the axis 
of the gear. The number of teeth A', and the pitch di- 
ameter D, have no relation to the helix angle. The 
chordal thickness t c and the tooth space chordal 
width s, refer to the pitch circle as shown in Fig, 




9-21. In a section perpendicular to the tooth, the 
following dimensions are defined: The normal pres- 
sure angle <j>, the normal diametral pitch P n , the 
normal chordal thickness t n , the normal tooth 
space chordal width s n , and the backlash B. 0, P n , 
and B are selected from the conventional values as 
for a spur gear. In a section perpendicular to the 
gear axis are defined the tangential pressure angle 
<t> t , the diametral pitch P, " and the tangential 
backlash B t . The following relations hold: 

. , tan 

tan ffl. = 

' cosct 

P — P n cos oe = j: 

B f = 

cos a 

ametral pitch, 60 teeth and minimum backlash. A 
circular locator (spring roll or ball) must be designed. 

, tan 20 0.36397 

tan <p f = 5 — ; = 

r cos 33 33 0.83340 

= 0.43673 

O - _ f _ _ it 

r = 23 35 33 

P = 6cos33°33' = 5 


D — — — 12 inches 

n 0.030 

J=— g— = 0.005 inch 

^ = cos33°33' = - 006inCh 

, 90 , 0,006 ., „,„ o , „ 
^ = 60 + 2^T2 X57 ' 3=1 3052 

s n = s cos 0£ 

The included angle a for a "conjugate" (imaginary) 
V-section in the plane perpendicular to the gear 
axis, is calculated from the spur gear formulas and is 
converted to the included angle a' for the actual V- 
section (in the plane perpendicular to the tooth) by 

a a 

tan -_- — tan — cos a 

Dimensions J? and D a relating to a pin or ball 
centralizer are calculated as before, by substituting 
a and s n for a and s. 

Example- A helical gear with 33°33* helix angle and 
20 degree normal pressure angle has 6 normal di- 

f = <23°35'33" + 1°30'52") = 25°6"25" 
tan ~= tan 25°6'25" X cos 33°33' = 0.39052 

s = 12sinl°30'52" = 0.3172inch 
s n = 0.3 1 72 cos 33°33' = 0.2643 inch 

R = 

2cos21 19'55' 

= 0.1419inch 

D = 12cosl o 30'52" + 

2X0.1419(1 +sin21°19'55") 

= 12.3828 inches 



Clamping Elements 


Here, as in other cases within shop terminology, 
the word "clamp" has more than one meaning; it is 
the common designation for all devices by which a 
part is secured in a fixture against the acting forces 
and it is also the name for a specific type of holding 
device described as a "strap." While the most im- 
portant phase in fixture design is the locating phase, 
the clamping phase takes its place as second in im- 
portance. Its technical importance lies in the fact 
that clamps must generate and direct the acting 
forces in such a way that the part is securely locked 
in place without suffering injury in the form of 
elastic distortion (springing). The economic aspect 
of clamping is just as significant, because clamping 
and releasing the part absorbs a portion, perhaps the 
largest portion, of the total operating time. There- 
fore, clamps must be designed for safe and fast 

To accomplish this, clamping devices have been 
developed in an extremely wide assortment of di- 
verse types and details. Almost all of them are now 
commercially available and are, to some degree, 
standardized." The fixture designer's task is no longer 
to invent and design new clamping components, but 
to select the right type and size from those on the 
market. A comprehensive display of clamping com- 
ponents is found in Chapter 17 and the present dis- 
cussion is confined to an explanation of principles, 
the description of representative applications, and 
references .to details. 

The basic types of clamping devices are: screws, 
straps, wedges, cams, toggles, and rack and pinions. 

Racks and pinions are discussed in Chapter 21. 
The action of most clamps is based on friction, and 
they are actuated either manually, pneumatically, or 
hydrauiically. Predominantly, clamps are of the 
strap type; structurally, they are modifications of 
the simple beam. 

The Mechanics of Wedges 

The action of wedges, screw threads, and cams are 
all based on the same friction relations. A screw 
thread is a wedge rolled around a cylinder. A flat 
cam is a wedge folded around a circle. To clamp a 
workpiece by means of a wedge requires, in the 
simplest case, that one side of the wedge bears 
against the work surface, as shown in Fig. 10-1 a, 
while the other side is supported by a surface in the 
fixture. With the coefficient of friction £<, the forces 
on the two sliding surfaces are P and fiP, and it re- 
quires the force shown as F t to insert the wedge. 
To calculate F x the forces P are resolved into their 
components with respect to the axis of the wedge, 
as shown in Fig, 10-lb, Equilibrium requires that 
the sum of all force components parallel to the 
wedge axis equals zero, from which we get 

Fi = 2P sin — + 2 M P cos~ 

2 ' ~ r " ""2 
+ n cos ■ 

sin — + n cos -z I 

To withdraw a wedge that holds the work with 
a pressure P requires a force F 2 found by 

F 2 =-2i > sin^+2M^cos- 

Since the parenthesis includes a minus term, it may 
become zero, which gives 

a a 

— sin^r + fX cos -r = 

tan^ = P 




Ch. 10 

Table 10-1. Coefficients of Friction ij. for Wedges, 
Cams, Pivots and Bearings Made of Hardened Steet 


sin £ 
Fig. 10-1. The mechanics of the wedge. 

The result, F? = 0, means that the wedge is no long- 
er self-locking. The condition for self-locking de- 
pends strongly on the coefficient of friction, but so 
do the values of F\ and F 2 . The coefficient of fric- 
tion depends not only on the contacting surfaces, 
but also on the pressure (ij increases with increasing 
unit pressure) and the operating conditions. Every- 
thing in a machine shop carries an oil film (unless it 
has been recently degreased). This condition is as- 
sumed for clamping operations. While clamped 
under pressure the surfaces have a tendency to break 
through the film and "bite," and the coefficient of 
friction for release is significantly higher. On the 
other hand, if exposed to vibration, the surfaces 
may work loose resulting in a reduced coefficient of 
friction. Recommended values of (i for wedges, 
cams and pivots and bearings are given in Table 10-1. 

Wedge or Cam Acting on 

Coefficient of Friction jU t 



Hardened steel 

0.19 to 0.20 

0.19 to 0.20 

Machine steel 

0.17 to 0.19 


Cast iron 

0.15 to 0.17 

0.17 to 0.19 

Aluminum alloy 

0.17 to 0.18 

0.18 to 0.20 

Laminated plastic 

0.12 to 0,16 

0.15 to 0.18 

Pivots and bearings 

Coefficient of Friction ji 2 

Good Condition 


0.03 to (i.06 

0.10 to 0.1 S 

Self-locking wedges and cams are made with tapers 
from l:2G(a = 2°52') through 1:10(<* = 5°44')and 
up to a wedge angle of 7 degrees (taper 1:8.18). 
For wedges exposed to vibrations it is recommended 
that the taper should not exceed 1:15 (a = 3°47'). 
Wedges designed within these limits have a short 
operating range and are only used on parts with 
fairly close tolerances. When the wedge is posi- 
tively held in place, larger wedge angles can be used. 
Wedges that actuate plungers or other movable com- 
ponents are made with al least 1 5 degrees (taper 
1:3.8) and up to 45 degrees wedge angle (plungers 
with ends cut under 45 degrees). 

Example - Assume a 7-degree wedge acting on a cast 
iron part with fi = 0.15 for clamping and p = 0. 1 8 
for release. 

F } = 2i>(sin3 1/2° + 0.15 cos 3 1/2°) 
= 0.422/" 

The pressure on the part is, in this case,«s2 1/3 
times the applied force. 

F 2 =2P(-sin3 1/2° + 0.18 cos 3 1/2°) 
= 0.237F 

The force to release the part is, in this case, approxi- 
mately one-half of the clamping force. The wedge is 
self-locking for coefficients of friction down to 

ju = tan 3 1/2° as 0.06 

Ch. 10 



The Mechanics of Screws 

When a screw is used as a clamping device, the 
clamping force P on the work, or on a strap, is de- 
veloped by application of torque T to the head of 
the screw or to the nut (see Fig. 10-2). In addition, 
the torque must overcome friction on the thread 
and under the head. All forces can be calculated by 

Fig. 10-2. The mechanics of the screw thread. 

use of the wedge formulas with the helix angle and 
the thread angle taken into account. Detailed analy- 
sis of the system is available from text books on 
machine elements. However, considerable simplifi- 
cation is possible, because the standardized screw 
thread systems have constant thread angle and near- 
ly constant ratios between the significant dimen- 
sions (nominal, pitch, and root diameter, etc.). With 
a coefficient of friction of 0.15 as representative of 
average workshop conditions and nominal screw di- 
ameter D, the formula connecting torque T and 
clamping pressure P is 

T= 0.2 DP 

(H = 0.15) 

With well cleaned and lubricated screw threads, 
the coefficient of friction can go down to around 
0.10, which is about the best that can be expected 
under workshop conditions. Values for T for various 
values of 11 are 

T= 0.164 DP 
T =0.139 DP 

0i = 0.10) 
(li = 0.08) 

The formulas as written are valid for coarse threads. 
For fine threads T is 3 to 5 percent less; a rather 
surprising result. One would expect a greater dif- 
ference, but the gain in mechanical advantage is 
partly absorbed by the increased friction losses. All 
standard UNF, UNC, and all of the machine screw 
series of screw threads are designed to be self-lock- 
ing and no further analysis is required with respect 
to this point. 

Manual Forces 

The torque on a clamping device is produced by 
manual force. Mechanical clamping devices do not 
use screws but apply their pressure directly, or 
through a simple linkage. 

The force, that can be provided by an operator, 
is not a constant quantity and cannot be calculated 
as are mechanically generated forces. Nevertheless, 
numerous attempts have been made to measure 
them: their average and mean values and their upper 
and lower limits. Most of these have been directed 
towards the manipulation of machinery controls in 
general and have little bearing on the actuation of 
fixture clamps with the exception of a few results of 
a general nature. Test results show acceptable cor- 
relations and normal distributions around a mean 
value; the maximum value is 3 to 4 times the mini- 
mum value, and the minimum value is 0.45 times 
the mean. 

Data for two operations are needed in manual fix- 
ture design: to pull (or push) a lever with one hand, 
and to turn a knob. The lever can be a wrench, a 
handle on a nut, or a cam lever. From evaluation of 
laboratory data it appears that a mean value for 
pulling a lever with one hand is 90 to 110 pounds 
(400 to 490 N). Data of this nature, however, must 
be taken with some reservation. The force a man 
exerts during a laboratory test is not necessarily the 
same as that which he would use while on his job, 
where he (unknowingly) is influenced by his physi- 
cal condition, his willingness, the frequency of the 
effort, his degree of fatigue, and his age. It was 
found in one case that the maximum one-hand pull 
was 125 pounds (556 N) for a 25-year-old man and 
1 03 pounds (458 N) for a 60- year-old man. It is not 
known to what extent the equipment used in the 
various tests is representative of common shop 
equipment. The length of the lever is of importance 
because it affects the mode of the grip and the ge- 
ometry of the motion. A rule-of-thumb, applicable 
to levers of up to 8 inches (200 mm) in length, gives 
2.8 pounds per inch (5 N per cm) lever length as the 
average applied force in one-hand operations. 



Ch. 10 

Laboratory results agree well on the significance 
of fatigue. The force applied repeatedly by one 
hand should not exceed 30 to 40 pounds (135 to 
175 N). European design practice uses 145 to 175 
newtons (33 to 40 pounds) as the force applied to a 
lever. American practice is to take 30 pounds (135 
N) for calculating the operating clamping force, but 
to calculate dimensions for an occasional maximum 
force of 90 pounds (400 N). 

The turning of a knob is clearly a one-hand opera- 
tion. The torque that can be exerted depends on the 
shape and dimensions of the knob; the maximum 
torque in repeated application can be taken as 47.5 
inch-pounds per inch (211.3 N mm per mm) knob 
diameter for round and four-lobe hand knobs. 

The above are design values which means limiting 
values, and not necessarily operating values, and 
must be applied with due consideration of all facts 
including the dimensions of the equipment. No 
workman would use from 30 to 90 pounds on a 
wrench to tighten a 3/8-inch screw. The operator 
adjusts his force to the job at hand; he pulls the 
wrench until he "feels" that the screw is "tight." 

The results of a test program performed under 
actual shop conditions 1 are listed in Table 10-2. The 
operator was instructed to "pull normally until the 
screw is tight." Screw thrusts were measured by a 
load cell. Threads were clean and normally lubri- 
cated. Data of this type are closer to reality and 
more reliable for use in design than data based on 
more or less uncertain assumptions. It is interesting 
to note the drop in thrust from the 5/8 inch- to the 
3/4- inch screw. It reflects the combined effect of 
larger pitch, lower mechanical advantage, more fric- 
tion loss, and greater physical effort required of 
the operator. 

The Mechanics of Cams 

A cam may be thought of as a wedge folded 
around a cylinder. Figure 10-3 shows a cam actu- 
ated by a force F, on a handle of length I, measured 
from the pivot center C, and exerting a clamping 
force P and a frictional force n l P at the point of 
contact D. These forces generate a reaction R with 
a frictional force /i 3 R on the pivot. Any point on 
the cam contour is defined by radius vector r and 
position angle 6 measured from the low point A. 
The location of D, relative to C, is defined by the 
initial eccentricity e and the height H, which vary 

Table 10-2. Clamping Force from Hand-Operated 
Screws— Experimental Results 

Screw Turned by 

Screw Thread 

Hand Knob 



Clamping Force, Pounds 

1/4 - 20 



5/16 - 18 



3/8 - 16 



1/2- 13 



5/8- 11 



3/4 - 10 






according to the cam geometry and as a function of 
6. The surface and lubrication conditions at D and 
on the pivot are different and are reflected by the 
two different coefficients of friction pn and fi 2 
(Mi > Mi ; see Table 10-1). 

Tomco, Inc., Racine, Wisconsin. 

''V T 

— ! — ! p 

Fig, 10-3. The mechanics of a cam of arbitrary contour. 

Ch. 10 



Assume, for a moment, that there is no friction; 
the acting forces are now F and P; the reaction R 
does not enter into the equilibrium equation which is 

as the theoretical condition for self-locking. For 
any practical application a safety factor (FS) is re- 
quired, defined by 

F L=Pe 

F„ e 

The pressure P exists only as long as the force F Q is 
maintained. When F is removed, P subsides. This 
situation is unusable and unrealistic. Frictions are 
always present. Their effect is to drastically increase 
F for a given or required P and to provide the pos- 
sibility for self -locking. 

With friction, the equilibrium equation for the 
clamping phase is 

Pe +HtP H + n 2 R r i =FL 

R is composed of contributions from P and F. 
Cam devices are designed so that P is much greater 
than F ( 1 to 20 times) and dominates the value of 
R which, as a good working approximation, can be 
taken as 1 .03 P. Then 

P(e+li 1 H+ 1.03M 3 r 2 ) = FL 

HiH +n 2 r 7 


With the assumption that the cam is clamped in a 
self-locking position with the force P, it is now de- 
sired to release the cam by applying a force F to 
the handle. This means that F changes direction 
and so do the friction forces fi^P and l* 2 R. By 
changing sign for these; terms in the equation it 

Pe—ti l PH-n 2 Rr 2 =—F'l 

F' —e+(i 1 H+ 1.03fJ 2 i-a 

P ~ L 

As long as the equation gives a positive F the cam 
is self-locking. The theoretical limit for self-locking 
is when F' = 0, which makes R ■= P and gives 

—e + iX t H + ti 2 r 2 =0 
Miff + Vl r 2 = 

where (FS) is taken as 1.5 to 2. For ft, it is recom- 
mended to use the lower range of values (column 
"clamping" in Table i 0-1). The friction losses are 
evaluated by the cam efficiency 

■ o 

e + HiH + l.03u 2 r 2 

The cam contour has five points of interest; the 
low point A, the high point B (dead center), the 
lower and upper limits E and G, and the midpoint 
M, of the operating range. The upper limit G must 
never get close to B because of the danger of the 
cam snapping through in case of overload. This 
danger increases as the cam and its pivots and bear- 
ings wear. Safety against snap-through is obtained 
by proper selection of the location of M and the 
length of the operating range EG, as measured on the 
contour, or expressed as 2j3 in angular measure. 
Recommended values for (3 are from 30 to 45 degrees. 

Example- A cam clamp is designed with the follow- 
ing values: 

In English Units 

1 = 5 inches, r 2 = 0.25 inch 

H = 0.84 inch, e - 0.081 inch 

/i! = 0,18,ju 2 = 0.05, required P = 600 pounds 

600 5 __ 

F "^ 0.081 + (0,18X0.84) + (1.03X0.05X0.25) 

F = 29 A pounds 

(0. 1 8 X 0.84) + (0.05 X 0.25) _ 

(FS) = 
efficiency = 



■= 2.02 

0.081 + (0.18 X 0.84) + (1.03 X 0.05 X 0.2S) 
X 100 = 33.0% 



Ch. 10 

In SI (Metric) Units 

L = 125mm r% — 6mm 

ff= 21mm e = 2mm 

Hi = 0.18, ju 2 = 0.05, required P = 2670N 
2670N 125 

The slope of the tangent and the normal to the 
curve is determined by 

F ~2 + (0.18X 21) + (1.03 X 0.05 X 6) 
ItSF = 2670 X (2 + 3.78 + 0.3 1 ) 
I25F= 16260 
F= 130N 
FS = (0.18X21) + (O.Q5X6) ^ % Q4 

efficiency = 

2 + {0.18 X 21)+ (1.03 X 0.05 X 6) 
= 32.8 percent 

X 100 


Fixture clamping cams are formed as Archimedes 
spirals or as circular eccentrics. 

The fundamental equation for an Archimedes spi- 
ral (see Fig. 10-4) is 

where the angle 8 is in radians, and / is the lead, that 
is, the increase in radius vector r for one revolution. 

Fig. 10-4. The mechanics of the Archimedes spiral cam. 

(The angle equivalent of one revolution is 8 = 1m 
radians which, when substituted in the above equa- 
tion, gives r = I). The rise is the increase in r along a 
given length of contour. The rise over the entire 
operating range is the "throw." 

tan a — 



The angle a is equivalent to a wedge angle. While / 
is constant, r and a vary with 6; a increases with I 
and decreases with increasing r. For fixture clamp- 
ing cams the variation of the angle a over the useful 
length of the cam is small, usually only around 1 de- 
gree. It is not difficult to make the cam self-locking 
over a wide operating range. There is no low or high 
point on a spiral. 

For practical applications it is convenient to meas- 
ure 8 from a fixed point A with radius vector t . 
This modifies the equation to 

W « + S« 

At an arbitrary point D we have 

e = r sin a 
H = r cos Of 

There are several empirical recommendations for 
the selection of the lead / for fixture clamping cams. 
One such recommendation says 0.O01 inch (0.025 
mm) per degree per inch radius which, on a 2-inch 
(50-mm) cam radius, gives / = 0.72 inch (18.3 mm) 
and tan a = 0.0573. This cam is self-locking for 
ti = 0.06. Another recommendation says that the 
throw over 90 degrees shall be 1/6 times the radius. 
This gives f=2/3r and, when applied to a 1 1/2-inch 
(38-mm) radius, tana = 0.106. This cam is intended 
to be self-locking for /J = 0. 1 . 

Eccentric Cams 

The circular eccentric cam (see Fig. 1 0-5 ) is char- 
acterized by the eccentric radius R and the fixed ec- 
centricity E. This cam has low and high points A 
and B. The point of contact D, is defined by r and 
8 as before, height H and eccentricity e have, like- 
wise, the same meaning. For calculating // and e it 
is convenient first to find angle \j/, which is done by 

sin jg 

sin (180-0) R 

sin $ = — sin 6 

Ch. 10 



in Table 10-3. The location of these ranges is shown 
in Fig. 10-6. 


i \£/N 

E \ 





i v \ \ * 

r\ i 




Fig. 10-5. The mechanics of the eccentric cam. 

An auxiliary angle is needed temporarily; it is 
found by 

9O + + (l8O-0) + ^= 180 
= 0-^-90 


tf - R = E sin = — £ cos (0 - 4>) 
H = R-Ecqs(0~t1j) 
e = E cos = £ sin (0 — i/0 

It should be noted that e is independent of R and 
depends only on the fixed eccentricity E and the 
angle. 8 — $ is the cam rotation angle. ^ is always a 
small angle and f = degrees at 8 - degrees and 
Q = 1 80 degrees, e is zero at degree and 1 80 de- 
gree cam rotation and has a maximum e = E at 90- 
degree cam rotation. $ has here its maximum value 
determined by 

Fig. 10-6. The location of self-locking ranges on an eccentric 

Table 10-3. Circular Eccentric Cams 
Self-Locking Operating Range for fi = 0.1 


Self- Locking Operating 


Range Angle 6,dcg, 


Oto 17, 

163 to 180 


to 23, 

157 to 180 


to 30, 

150 to 180 


to 37, 

143 to 180 


to 44, 

136 to 180 


to 53, 

127 to 180 


Oto 64, 

1 16 to 1 80 


to 85, 

95 to 180 


Oto 180 

sin 4/ = ^ 

Since tan tf> is the slope of the tangent, it follows 

that the cam is least likely to be self-locking in this 

position, or, if it is self-locking in this position, it is 

always self-locking, 

For (1 = 0. 1 a cam is self-locking for — = 0.0996 1 

or -7T = 20.8. The preferred operating range is sym- 

metrical with respect to the position for maximum e. 

The self-locking range for other values of -=• is listed 

The Mechanics of Toggle Clamps 

Toggle clamps are linkage operated clamps and are 
based on the same kinematic principle as the eccen- 
tric clamp but with widely different dimensions of 
the moving parts. They possess enough elastic flexi- 
bility to allow the actuated link to pass through dead 
center. A positive stop just beyond that point de- 
fines the locking position and the resistance at dead 
center secures the link in that position. 

At dead center the initial eccentricity e equals 
zero and, in the absence of friction, the mechanical 
advantage equals infinity. This holds for any eccen- 
tric cam, including toggle clamps. Mathematically 



Ch. 10 

this means that the clamping pressure P becomes in- be generated by an infinitely small actuating force 
finity for any finite value of the actuating force F, F. In reality, we have neither infinitely large nor 
or conversely, that a finite clamping pressure P can infinitely small forces, and the mathematical model 

- -6P 



Fig. 10-7. The action of the toggle clamp, a. A toggle clamp of the push-pull type in the open and closed position; 
b. The force system in the toggle clamp at the moment of clamping. 

Ch. 1 



means simply that a small (but finite) actuating force 
Fcan generate a large (and still finite) clamping pres- 
sure P, as indicated in Fig. 10-7a. 

The technical interpretation of these seemingly 
paradoxical statements is, that a perfectly rigid ec- 
centric cam actuated by a finite force can operate 
only to the point of positive contact with the part 
to be clamped. The clamping is effective if this 
point is near dead center. If it is at dead center, 
then the clamping can only be effective when all di- 
mensions in the system are mathematically accurate; 
a completely unrealistic assumption. In mechanical 
language, the system is statically indeterminate; the 
height of the part to be clamped is the redundant 

Actually, there are only two possibilities, either the 
cam clamps before dead center or it slips through. 
Toggle clamps are not designed with the same rigid- 
ity as solid cams, and their inherent flexibility dras- 
tically changes the force conditions during the clamp- 
ing operation. After contact has been established 
(a short distance before dead center) pressure builds 
up and the entire toggle mechanism is elastically de- 
flected. The maximum deflection and correspond- 
ing maximum pressure occur simultaneously as the 
actuating link passes through dead center; the pres- 
sure is regulated by means of an adjusting screw in 
the pressure pad or elsewhere in the linkage system. 
The actuating force required for moving the link 
from the point of initial contact to dead center is 
small, because the mechanical advantage is large; on 
the dead center, where the mechanical advantage is 
infinity, the actuating force would vanish if the 
mechanism were frictionless. The actuating force 
necessary on dead center is the force required to 
overcome the frictional forces on the pivots and 
bearings, and is calculated as follows: Maximum 
pressure P produces friction forces as shown in Fig. 
10-7b. Since P is many times F (example: F = 15 
pounds, P = 1000 to 2000 pounds), it is permissible 
to ignore any transverse reactions from F. The actu- 
ating force is transmitted to the pressure link by a 
direct force F t , so far unknown, and a friction force. 
First, consider the forces on the pressure link. Tak- 
ing moments about the center of the right-hand 
pin gives 

F l B + l±P(,B~R)=pPR 
F l B = nP(2R -B) 

Fj may well come out negative. This is not disturb- 
ing; it means simply that the friction force is signifi- 
cant in the transmission of the actuating force. Now, 
consider the forces on the operating lever. Taking 

moments about the center of its bearing pin gives 
FL =F t A +^PR +fiP(A +R) 
= 2 t iPR A ~=2nPR{j+l) 

Note that the ratio of F and P does not depend on 
the individual lengths A and B, only on their ratio. 

In English Units 

Example- A toggle clamp has: L — 12 inches, 
A = 3/4 inch, B = 2 inches and R = 1/4 inch. 
Assume fi = 0. 1 5, Then 

FX 12 = 2X 0.15 X?X 1/4 X (3/8+ 1) 

-= = sa»l 16 


In SI (Metric) Units 

Example- A toggle clamp has: L = 300 mm, 
A = 19 mm, B = 50 mm and D = 6 mm. 
Assume fx = 0. 1 5, Then 

FX 300 = 2X 0.15XFX 6X 

-= 120.8 ss 121 

19 + 50 

The Mechanisms of Beams 

Straps are beams and are loaded in bending. The 
loads are the applied force F, the clamping force 
P and a reaction R at the point of support. The ap- 
plication of straight straps as clamping elements in 
fixtures includes the five different force arrange- 
ments shown in Fig. 10-8, a through e. The angle 
strap is shown in f. 

In the design and stress analysis of a strap clamp it 
can be assumed thatF is known, and it is required to 
calculate the applied force F, and the maximum 
bending moment M, which always occurs at the load 
that is located in the middle part of the strap. The 
formulas for F and M are: 

Case (a) 

F~ L 2 

M = RLi =P(L 2 -£,) = F- 

I. ) {L 2 -LO 










J R 1 R r 


t^ztz^zi ' 

Ch. 10 

T L L i i H pT— l 2— I M £ 


Fig. 10-8. The mechanics of the beam type strap and angle clamp. Applied force; F, F lt F 7 ; clamping pressure: P; 
support reaction: R, R 1t R 2 . 

Case (b) 


M = F 

L V L 2 

Case (d) 

F L 2 

M = FLi =PL : 

Case (c) 

Case (e) 

Fx+F 2 l 

F L 2 

(Li L 2 )L 2 

Ch. 10 



( use ff) 


M = FL t =PL 3 

Cases (b) and (c) are normally used with L] = L 2 - 
All six cases can be screw, cam, or mechanically 
actuated. However, screw and cam actuation is more 
common with cases (a), (b), (c), (d), and (f); me- 
chanical actuation is more common with cases (d), 
<e), and (f). 

In case (a) the force ratio (the mechanical advan- 
tage) is less than 1 ; only cams (d), (e), and (f) have 
the possibility of a mechanical advantage greater 
than 1 , It is usually desired to make the mechanical 
advantage as large as possible. Therefore, in case (a), 
F should be as close to P as possible, and in cases (d) 
and (e), F should be as close to R as possible. 


Table 10-4. Representative Average Operating Time 
for Clamping Devices 

(Time is for clamping only; does not include release time) 

Type of Clamp Operated 



Socket head screw 


Hexagonal nut or set-screw 


Screw jack 


Bar knob, bar screw 


Spoke nut 


Hand wheel 


Backup screw (on a strap) 


Star-shape or other form of hand knob 


fixed jack 


Hand-operated cam 




Sliding strap and other quick-acting 

clamps of various design 

0.02 to 0.04 

L- and T-pins (for locating through a hole) 

t /2-inch diameter, and up 


less than 1/2-inch diameter 


The clamping devices used in connection with jigs 
and fixtures may either clamp the work to the jig 
or the jig to the work, but very frequently the 
clamps simply hold a loose or movable part in place 
in the jig, the part can then be swung out of the way 
to facilitate removing and inserting work in the jig. 
The work, in turn, is clamped by a set-screw or other 
means passing through the loose part, commonly 
called the "leaf." 

M ost clamping devices have some basic features 
in c omm on: 1, They are made from high-strength 

material; 2. Contact surfaces are made wear-resist- 
ant; 3. They are designed for quick operation; and 
4, When released they can be moved clear of the 
working area. 

When making a sRlp.rtion_ between two or more ili£- 
ferent types of clamp' the^ fixture designer must weigh 
savings mjaperarirtgjime again^Tiabricating (or pur- 
chasing) cost. Time-study data are valuable for this 
pur pose, if available; if not , comparative TstinTates 
can be based on average empirical values for the" 
com mon~ claTnpfhg operaTionsT" XT isToTsuch v alues 
is given in Table-1 0-4. Actuating a valve or "aswifch 
for an air or hydraulically powered clamp takes about 
25 percent of the time required for manual clamping 
of the same part, Cams are faster to operate than 
screw clamps, but are also more expensive. Cams 
are preferred for large series and short operations. 
With operations lasting more than 5 minutes, the ad- 
vantages of the cam clamp become insignificant. 

Example— An operation of 5 minutes duration re- 
quires either a screw clamp costing $ 1 2.00 or a cam 
clamp costing S50.00. Clamping time for the screw 
clamp is 0.16 minute, for the cam clamp, 0.07 min- 
ute. Release time is 50 percent of clamping time. 
Labor plus overhead is S10.00 per hour. 

Higher cost of cam: $50.00 - SI 2.00 = S38.00 

Savings in time per part with cam operation: 
(0.16 -0.07) X 1.50 = 0,135 minute 

Savings per day in $ (disregarding losses, "breaks," 

^r^X^jp-X $10.00= $2.16 

3 Ott 

Time required for break-even: 

_ " = 17.6 « 18 working days 
2. 16 

Production required for break-even: 
18 X 8X^= 1728 parts 

Clamping Screws 

Clamping fasteners (screws and nuts) fall into two 
groups; fasteners turned with a wrench, and hand- 
tightened fasteners. Fasteners for wrench operation 



Ch. 10 

are set-screws with hexagonal head, collar-head 
(square-head) screws, and socket-head screws. Nuts 
used are hexagonal nuts, hexagonal flanged nuts, and 
acorn nuts. Collars, flanges, and washers are used to 
spread and reduce the pressure. Spherical washers 
are used to equalize the pressure when clamping on 
irregular contacting surfaces. Acorn nuts are used to 
protect the thread against damage and dirt. The 
height of hexagonal nuts should be 1 1/2 times stan- 
dard height to reduce the load on the thread. 

The acorn nut can be modified as shown in Fig, 
10-9. The purpose of this design is to permit lifting 
the wrench off the "hex," and moving it' back for a 
new grip. The round part of the nut serves to keep 
the wrench in place to be slipped back onto the hex- 
agon nut, while the pin at the top of the nut makes 
the wrench an integral part of the fixture so that it 
cannot get lost. 

Fig. 10-9. Acorn nut for clamping screw, modified for 
retention of wrench. 

Machine steel of 60,000 psi (414 N/mm 2 ) tensile 
strength is satisfactory in cases where the load is 
light; however, most fasteners for clamping purposes 
are made from steel with 120,000 to 150,000 psi 
(830 to 1035 N/mm 2 ) tensile strength. Socket-head 
screws, are usually made from a 185,000 psi (1275 
N/mm 2 ) tensile strength steel. They are now 
widely preferred because of their strength and reli- 
ability, the fact that they require less space than 
older types of fasteners, and because they are safer 
in operation; the wrench cannot slip when it is prop- 
erly inserted in its socket. 

Hand-tightened fasteners are designed to provide a 
good grip for the hand and at the same time elim- 
inate the need for a wrench, which could result in 
overloading the clamp or the part. Simple and inex- 
pensive hand-grip fasteners that do not require pur- 
chase of commercial items are shown in Fig. 10-10, 

C £I> 


Fig. 10-10. Hook bolt and hand-actuated fasteners. 

A special type of bolt used in drill jigs is the "hook" 
bolt shown at left in the illustration. It is very cheap 
to make and easily applied. The bolt A, passes 
through a hole in the jig (having a good sliding fit in 
this hole) and is pushed up until the hook or head B, 
bears against the work; the nut is then tightened. 
When great pressure is not required, a thumb- or 
wing nut provides a better grip and more satisfactory 
means than the knurled nut (shown at right), for 
tightening down upon the work, and permits the 
hook-bolt to be applied more readily. When work is 
removed from the jig, using the hook-bolt clamping 
device, the nut is loosened and the head, or hook, of 
the bolt is turned away from the work, thus allow- 
ing the workpiece to be taken out and another placed 
in position. Figure 10-11 shows an application of a 
bent hook-bolt. Generally speaking, the type shown 
in the previous illustration is better suited to its pur- 
pose, as the bearing point on the work is closer to 
the bolt body and it can be drawn more tightly. 

Fig. 10-U. Clamping with hook bolts. 

A screw with a pin through the head can be used 
for light clamping, but is inconvenient to work with. 
More convenient is the knurled circular nut shown 
previously, and also screws with knobs of the same 
type. Because of the better grip obtained, greater 
force can be applied by the use of wing-nuts, wing 
screws, and hand knobs of various shapes, used as 
screw heads and nuts. An application is shown in 
Fig. 10-12. These fasteners are all commercially 
available. Hand knobs come with 3, 4, and 5 lobes 
and in a range of sizes. The amount of force which 
the operator can apply to the knob increases with 
its size. With respect to shape, test results show that 

Ch. 10 



mended for general use as the screws are severely 
loaded in bending, but as a last resort, it may be ac- 
ceptable in cases where only light cuts are taken. 

Fig. 10-12. Clamp tag screw with hand knob. 

the 4-prong knob best fits the anatomy of the hu- 
man hand. 

If screws are to be firmly tightened without the 
use of a wrench, the method of using a pin through 
the screw-head can be used on large fixtures. The 
pin is 1/2 inch (13 mm) in diameter, or more, and 
requires the use of both hands (see Fig. 10-13). A 
more sophisticated version is the speed nut, a com- 
mercial component consisting of a nut with one or 
two arms terminating in ball knobs. This is for two- 
hand operation and permits fast spinning and hard 

Fig, 10-13. Clamping screw with pro handle. 

The use of manually operated screws, however, 
does not completely eliminate the possibility of over- 
loading the clamp. Full safety against overload can 
be achieved by the use of torque head screws. The 
screw has a knurled head with a built-in spring-loaded 
clutch that slips automatically when the maximum 
load is applied. Many screw fasteners are, or can be, 
provided with swivel ing pads for even pressure dis- 
tribution and protection of the surface of the part. 

Examples of the direct application of screws to 
substitute for the use of clamps are shown in Fig. 
10-14. The method shown in diagram a is simple 
and self-explanatory. The method shown in diagram 
b requires screws with conical tips and is used in 
milling fixtures for light milling operations; the part 
is clamped horizontally and vertically at the same 
time. The method shown in diagram c is not recom- 

Fig. 10-14. Direct clamping with screws: a. The general 
method; b. An inexpensive method suitable for 
light-duty operations; c, A clamping method 
suitable for light-duty only -not recommended 
for general use. 

Jack Screws 

The o rdinary jack screw is frequently employed 
as a "supporting device in ordinary setups on a ma- 
chine tool, but rarely in fixtures as it is a loose part 
artd~ls~quite apt to get lost. In Fig. 10-15 two simple 
deviceTamhbwn, that work Oft the same principle 
as the jack screw, but they have the advantage of 

Fig. 10-15. Swing jack screws. 



Ch. 10 

being connected to the jig by pin B. At A, a set- 
screw screws directly into the end of the eye-bolt, 
and at C, a long square nut is threaded on the eye- 
bolt. These nuts must be of a particular length, and 
made especially for this purpose. The eye-bolts are 
fastened directly to the wall of the jig, and the set- 
screw, or nut, is tightened against the work. The 
eye-bolt can be set at different angles to suit the 
work, thereby providing a means of double adjust- 
ment. This is a very convenient clamping device 
which works satisfactorily and can be easily swung 
out of the way. Several types of jack screws for 
permanent mounting in fixtures are commercially 


The simplest form of clamping device is the strap. 
A number of different forms of commonly used 
straps are shown in Figs. 10-16 and 10-17. Perhaps 
the most common of all is that shown in diagram 
1 0-1 6a. It is simple, cheap to make, and, for most 
purposes, it gives satisfactory service. The clamp 
shown in diagram 10- 16b is practically the same 
principle, but with several added improvements. It 
is recessed at the bottom for a distance b, to a depth 
equal to a, so as to give a bearing only on the two 
extreme ends of the clamp. Even if the strap should 
bend somewhat because of the pressure of the screw, 




n m 

■£4 ft 

Fig. 10-16. Strap clamps. 

it would be certain to bear at the ends and exert the 
required pressure on the object being clamped. The 
strap is also provided with a ridge at D, located cen- 


Fig. 10-17. A self-adjusting strap clamp. 

trally with the hole for the screw. This insures an 
even bearing of the screw-head on the clamp, even 
if the two bearing points at each end of the clamp 
should vary in height, as illustrated in Fig. 10-17, 
left. The clamp at a in Fig, 10-16 would not bind 
very securely under such circumstances, and the col- 
lar of the screw might break off as the entire strain, 
when tightening the screw, would be put on one side, 

A further improvement in the construction of this 
clamp may be had by rounding the underside of the 
clamping points^ (Fig. 10-17, right). When a clamp 
with such rounded clamping points is placed in a 
tilted position it will bind the object to be held 
fully as firmly as if the two clamping surfaces were 
in the same plane. 

The hole in these straps is very often elongated, as 
indicated in Figs. 10-16 a and b by the dotted lines, 
which allows the strap to be pulled back far enough 
to clear the work; making it easier to insert and re- 
move the piece to be held in the jig. In some cases, 
it is necessary to extend the elongated hole, as shown 
in Fig. 10-1 6c, so that it becomes a slot (going all 
the way through to the end of the clamp) rather 
than simply an oblong hole. Aside from this dif- 
ference, the clamp works on exactly the same prin- 
ciple as those previously shown. 

To suit different conditions, instead of having the 
strap or clamp bear on only two points, it is some- 
times necessary to have it bear on three points, in 
which case it may be designed similarly to the strap 
shown in Fig. 1 0-1 6d. In order to get equal pressure 
on all three points, a special screw, with a half- 
spherical head may be used to advantage. The head 
fits into a concave recess of the same shape in the 
strap. When the bearing for the screw-head is made 
in this manner, the hole through the clamp must 
have a generous clearance for the body of the bolt. 

When designing ciamps or straps of the types 
shown, one of the most important considerations is 
to provide enough metal around the holes, so that 
the strap will stand the pressure of the screw without 
breaking at the weakest place, which, naturally, is in 
a line through the center of the hole. As a rule, these 
straps are made of machine steel, although large 
ciamps may occasionally be made of cast iron. 

Ch. 10 



At e and f in Fig. 10-16, bent clamps and their ap- 
plication to the workpiece are shown. These clamps 
are commonly used for clamping work in the planer 
and milling machine, but are also frequently used in 
jig and fixture design as well. 

Screws used for clamping these straps are either 
standard hexagonal screws, standard collar-head 
screws, or socket-head screws. When it is unneces- 
sary to tighten the screws very firmly, thumb-screws 
or screws with hand knobs are frequently used, 
especially on small jigs as explained earlier. 

The simple strap clarnp is now commercially avail- 
able in a large number of very sophisticated modifi- 
cations, comprising single- and double-end straps, 
center and rear-end applications of the clamping 
screw, and fixed and adjustable height end support. 
Without exception the strap has an elongated hole 
to permit withdrawal from the work area, and a 
spring to keep it in the lifted position when released. 
Strap clamps are also made with cam actuation in- 
stead of screw actuation for quick clamping, and 
also with automatic withdrawal from the work area. 

Strap Clamp Applications 

A simple and common tyne of fixture r equires 
clamping with one or two screws only at the center 
of the part, combined with easy access to the work 
area from above. The solution is a clamping screw 
(or screws) located at the center of a removable 
strap clamp, straddling the part. The strap, or clamp, 
is arranged as shown in Fig. 10-18, the screw passing 
through it at the center and bearing upon the work, 
either directly or through the medium of a collar or 
a swiveling pad, fitted to the end of the clamping 
screw. T his type of clamping arrangement is eom- 
mo nly used for holding work in a drill jig. The strap 

used in this type of arrangement can be improved 
upon by making it in one of the forms shown in 
Fig. 10-19. Here the ends of the straps are slotted 
in various ways, to make it easy to remove the strap 
quickly, when the work is to be taken out of the jig. 





-<E3 — f — ^ 


Fig. 10-19. Strap clamp with slots for easy removal. 

Another way of making the strap removable is to 
support it in grooves in the fixture wall so that it 
can slide when the clamping screw is released. Two 
ways of doing this are shown in Fig. 10-20, Shown 
in Fig. 10-20a is a strap that can slide lengthwise 
through slots in the fixture walls; the slots must have 
clearances below the strap to allow for passage of 
the screw. In Fig, 10-20b is a strap that slides in 
grooves in the fixture walls. 

~i r 





Rfe. 10-18. Strap clamp with center screw. 

Fig. 10-20. Strap clamps, sliding, for easy removal 

The clamping pressure can be transmitted to the 
center of the part by means of the strap, with or 
without a pressure pad. Examples are shown in Fig. 
10-21. Diagram a, shows a strap with slots for easy 
removal. The strap shown in diagram b, is clamped 



Ch. 10 




n [ 




H is 









i ; jF- 




^ i 





SI ! 


Fig. 10-21. Strap clamps with end screws. 

by means of swinging bolts, a principle that has nu- 
merous applications. The strap shown in diagram c, 
is clamped by means of bolts with C-washers under 
the nuts and large bolt holes. When the nuts are re- 
leased, the C-washers are easily removed, and the 
nuts can pass through the holes. The C-washers as 
shown are loose pieces. Loose pieces in a fixture are 
not desirable as they may be lost. In the present 
case this condition can be improved by the use of 
swinging C-washers. The C-washer is pivoted around 
a shoulder screw; when the nut is released, the C- 
washer is rotated out of engagement and the bolt 
can be withdrawn. Swinging C-washers with their 
pivot screws are standardized and commercially 

Angular Clamps 

Angular clamps are those that redirect the clamp- 
ing force. In most cases, the force direction is ro- 
tated 90 degrees so that, for example, a vertical 
clamping screw generatesa horizontal clamping force 
on the part. The clamping force is "'turned around a 
corner." Some of these devices have the double ef- 

fect of simultaneously clamping vertically and hor- 
izontally. Angular clamps are primarily necessitated 
by narrow space and restricted access conditions in 
the fixture. 

The basic, and simplest, form for an angular clamp 
is the angle strap shown in Fig. 10-8f. Mechanically, 
it is a bell-crank lever. There are several other means 
by which the clamping force can be redirected 90 
degrees or some other angle. The most important of 
these are the hinged strap (which is a modified ver- 
sion of the bell-crank lever), the linked strap, the 
combination of a strap with a wedge, and the com- 
bination of a strap with a plunger with a 45 degree 
inclined end surface. A widely used clamping de- 
vice, the "gripping dog" is, in effect, also a modifi- 
cation of the bell-crank lever. The number of pos- 
sible combinations is too large for a systematic clas- 


PjIflD o 

Fig. 10-22, Clamping with "gripping dogs.' 

Ch. 10 



sification, but the principles and their application 
will be demonstrated by representative examples. 
Irregularly shaped castings which must be ma- 
chined often present no apparently good means of 
holding by ordinary gripping appliances for drilling, 
shaping, or milling. In such cases, gripping dogs, as 
illustrated in Fig. 10-22, may be used. The basic 
type is the one shown in diagram a. The base block 
C is inserted in the T-slot of the machine table with 
a sliding fit and is prevented from slipping back- 
wards by the backstop F, which is firmly bolted in 
position. The base block is slotted to receive the 
jaw D which is fulcrumed on a cross-pin. In the tail 
of the dog a set-screw E is threaded. By turning 
this set-screw the jaw is caused to "bite" inward and 
downward at the same time, firmly gripping the 
casting and forcing it down on the table. Since the 
position of the backstop is adjustable, the same grip- 
ping dog can be used for castings of different sizes. 
A gripping dog can also be solidly mounted on a fix- 
ture plate, eliminating the need for a backstop. 

Gripping dogs have a tendency to draw the work 
down firmly and forcefully onto the rest-pins, or 
stops, and are useful in all classes of fixtures. A dif- 
ferent type is shown in diagram b. Care should be 
taken to see that the stop is pivoted above point A. 
Another, and more rigid, device is shown at c. Plun- 
ger .4, carried in plunger B, is forced down against the 
45-degree side of stop C, compressing spring D. A 
fixture that provides two clamps which exert a 
"down-and-in" pressure is illustrated in Fig. 10-23a. 
Slides B are equalized by strap C and ball-and-socket 
washers D and E. This fixture is useful for milling 
and profiling, as the clamps and stops are below the 
surface of the work. A modification of this fixture 
is shown in diagram b. It has two down-and-in 
equalized clamps for holding a round piece of bored 
work for a milling operation. Lever A is tapped to 
receive screw B, and the clamping pressure equalizes 
with lever C by means of rod D. Levers A and C im- 
part a down-and-in pressure to plungers E. Equali- 
zation is necessary as the work is already centered 
on a circular locator. This fixture can be applied 
to flat work. 

In the double-movement clamp shown in Fig. 
10-24, clamp A is carried by hinge B, pivoted at C. 
Screw E gives clamp A a down-and-in movement by 
means of a 45-degree taper on the contoured and 
hardened block D, which is also milled off at F to 

I 7 ig. 10-23. Clamping with "down-and-in" pressure. 


Fig. 10-24. A clamp with double movement. 



Ch. 10 

give the clamp sufficient movement to remove the 

The combined action of a strap and wedge simul- 
taneously produces horizontal and vertical clamp- 
ing. It is simple, strong, efficient, and fast, and has 
many applications. Figure 10-25 shows a fixture 

bears against and clamps the part, as shown in Fig. 
10-26b. 2 

An example of the use of plungers is shown in 
Fig. 10-27. Plungers A and B are built into the strap 
and are actuated by means of screw C with hand- 
knob D. In this way it is possible to reach, with the 

\"\g. 10-25. Double strap clamps with wedge action. 

where two such straps are clamped by one screw, 
also resulting in a centralizing action. The wedge 
end of the strap can bear against a mating surface 
on the fixture, as shown in Fig. 10-26a, 2 or the 
configuration can be reversed so that the wedge end 

Fig. 10-27. A plunger-actuated clamp for reaching into an 
otherwise inaccessible area. 

end of the strap, an otherwise almost inaccessible 
place in the workpiece. Another example, Fig, 
10-28, shows a small clamping device used when 
drilling rivet holes through beading A and plate B. 

Fig. 10-28. A small clamp with double action for drawing 
and clamping. 

Steel bracket C is fastened by screws to the side of 
the fixture; the front face of the clamp bracket is 
used as a stop for the plate and the beading; and 

Courtesy of E. Thaulow 
Fig. 10-26. Single strap ciamps with wedge action. 

E. Thaulow, Maskinarbejde (Copenhagen: G.E.C. Gad's 
Forlag, 1930) vol. II. 

Ch. 10 



clamp D, with a small hole drilled in one end, is 
fitted loosely in the milled slot in the bracket. The 
set-screw is located a little higher than the hole in 
the clamp and, by a few turns of the screw, the 
clamp is brought down against the work and forces 
the beading up against the stop, ready for drilling. 

Swinging Leaves 

A "leaf" or a "swinging leaf" is a hinged cover on 
a box-shaped fixture. It is normally used on drill 
jigs. The elementary principles involved in the 
swinging-leaf clamping construction are shown in 
their simplest form in Fig, 10-29. Loose leaves 



! i! 
i ii 

- 1 " 




fig. 10-29. The general principle of the swinging leaf clamp. 

which swing out, in order to permit the work to be 
inserted and removed, are usually constructed in 
some manner similar to that shown in Fig. 10-30 in 
which A represents the leaf, being pivoted at B and 
held by a pin at C, which goes through the two lugs 
on the jig wall and passes through the leaf, thus 
binding the leaf and allowing the tightening of set- 
screw D, which bears against the work. The holes in 
the lugs of the castings are lined with steel bushings 

Fig. 10-30. Swingleaf clamp with center screw. 

in order to prevent the cast-iron holes from being 
worn out too soon by the constant withdrawal and 
insertion of the pin. This kind of leaf, when fitted 
in well, is rather expensive, but is used not only for 
binding but also for guiding purposes, making a con- 
venient seat for the bushings. If leaves are fitted 
well into place, the bushings in the leaves will guide 
the cutting tools in a satisfactory manner. 

Another method of clamping down the leaf is 
shown in Fig, 10-31, in which A is a thumb-screw, 
screwed directly into wall B of the jig, and holding 
leaf C down, as indicated. The thumb-screw is a 
quarter-turn screw. To swing the leaf out, the screw 
is turned back about a quarter of a turn, so that the 
head of the screw stands in line with a slot in the 
leaf, which is both wide and long enough to permit 
the leaf to clear the head of the screw. This is a very 
rapid method of clamping, and is frequently used. 






Fig. 10-3 1. A quarter-turn screw used for locking a swinging 
leaf clamp. 

The lower side of the screw head will wear long be- 
fore the head finally turns in line with the slot when 
binding. It can then easily be fixed by turning off a 
portion of the head on the underside, so that it will 
bind the leaf again when standing in a position where 
the head of the thumb-screw is at right angles to the 
slot. The size of these thumb-screws is made accord- 
ing to the strain on the leaf and the size and design 
of the jig. 

The hinged or latch bolt, shown in Fig. 10-32 is 
also commonly used. Here,j4 represents an eye-bolt 
connected with the jig body by pin B. The leaf or 
movable part C of the jig is provided with a slot in 
the end for the eye-bolt, a trifle wider than the 

Fig. 10-32. An eye-bolt used for locking a swinging leaf 



Ch. 10 

diameter of the bolt. The threaded end of the eye- 
bolt is provided with a standard hexagon nut, a 
knurled-head nut, or a wing- nut, according to how 
firmly the nut must be tightened. When the leaf is 
to be disengaged, the nut is loosened just enough to 
clear the point at the end of the leaf, and the bolt is 
swung out around pin B, which is driven directly 
into lugs projecting from the jig wall; a slot being 
provided between the two lugs, as shown, so that 
the eye-bolt can swing out freely. At the opposite 
end, the leaves or loose parts of the jig swing around 
a pin, the detailed construction of this end being, 
most commonly, one of the three types shown in 
Fig. 10-33. It must be understood that to provide 
jigs with leaves of this character involves a great deal 
of work and expense, and they are used almost ex- 
clusively when one or more guide bushings are held 
in the leaf. 

Fig. 10-33. Details of leaf hinges. 

A hinged jig cover may also be conveniently held 
in place by a semiautomatic spring latch of the type 
shown in Fig. 10-34. The body of the jig is shown 


L_i_ i— 

_ i 









at A; the hinged cover at B. This cover swings on 
pivot C and drops onto latch D. In cases where the 
cover is merely used to carry bushings, a latch of 
this kind is entirely satisfactory, although it is not 
recommended for use on jigs where screws for hold- 
down the work are carried by the cover. To swing 
the cover clear of the work in the jig, latch D is 
pushed back in the direction of the arrow. After the 
cover has been raised, the latch springs back into 
place ready to catch the top of the cover automatic- 
ally when it drops back onto the jig, requiring no 
attention from the operator. 

When the leaf is used to transmit clamping pres- 
sure to the part, a latch-type lock is insufficient, and 
a screw-type lock is required. The quarter-turn 
screw (see Fig. 10-35) is used where the height of 



1 tee 


i k=.- 

i j 





Fig. 10-35. Leaf jig with quarter-turn screw. 

the part is within close tolerances so that no signifi- 
cant height adjustment is required. Should some 
height adjustment be necessary, such as in a case 
where the leaf clamps upon a rough surface, a clamp- 
ing screw with a longer travel can be used. A typical 
arrangement, using a swinging bolt, is shown in Fig. 
10-36, Since, in this case, the pari requires two 
clamping points; the clamp is pivoted so that the 
total clamping force is equally divided between the 

F(g. 10-34. Leaf jig with spring latch. 

Fig. 10-36. Swinging leaf clamp with an equalizer. 

Ch. 10 



two points. Most swinging-leaf components, includ- 
ing locking devices, leaves, and boxes are commer- 
cially available. 

"Clamping Mandrel-mounted Work 

The "natural" and convenient way of holding 
small parts that have been bored through and faced 
on the ends, is to mount them on a mandrel and 
make the mandrel an integral part of the fixture. 
When in position, the part is clamped. The simplest 
way of clamping it is with a washer and nut on the 
free end of the mandrel. This is a cheap and reliable, 
but slow, operation, because the nut must be run 
off and on each time a new part is placed in the fix- 
ture. The time required for running the hexagon 
nut on and off is saved as shown in the design in 
Fig. 10-37, using a quarter-turn knob. Stud B has a 
flat milled on both sides of its threaded end portion. 
The slot in knob A slides on over this flat and a 
quarter turn clamps the work. If the variation in the 
length of the work is not too great, this makes a 
rapid clamping arrangement. 



Fig, 10-37. Clamping on a mandrel with a quarter-turn knob. 

Figure 10-38 shows another means of clamping 
the same piece in which the variation in length of 
the work and the time required for turning the knob 
to match the flat on the stud has been considered. 
The slotted washer A and knob B are dropped over 
stud C; A is held against B, which can then be 
screwed up as freely as a solid knob. This can be 
used for a variety of bushings of various lengths; 
stud C being made to suit the longest piece of work* 
Using a square or Acme thread is recommended, 
since these have less tendency to tilt the nut than 
would a 60-degree thread. 

l'ig, 10-38. Clamping parts of varying length on a mandrel. 

Cams, Eccentrics and Toggle Clamps 

^ Clamping cams, eccentrics and toggle joint clamps 
differ from clamping screws in that they are more 
expensive to buy or make; they ar e faster to operate; 
a nd they have only a short effec tive clamping range. 
For a cam, the effective clamping range is 1/8 inch 
{3-mrn>, tor a toggle joint clamp it is 1/16 inch (l.S 
mm). As these mechanisms close, they tend to exert 
a slight lateral movement to the contacting surface. 
They are, therefore, used primarily to operate on 
another clamping member, such as a strap or leaf, 
rather than to clamp directly on the part. There are 
extensive analyses of the relative merits of the spiral 
cam and the circular eccentric cam, and industry has 
made its choice: the commercially available cam 
components are, as a rule, made with circular eccen- 
tric cams. There is little principal difference between 
this type of cam and an eccentric shaft; a toggle 
joint can be considered to be an eccentric device 
with a very large eccentricity. In addition, a toggle 
mechanism is fairly elastic, and this feature enables 
the toggle clamp to move beyond dead center when 

Eccentric shafts are often used for moving and 
closing a clamping strap. In Fig. 10-39 two applica- 
tions of the principle of the eccentric shaft are 
shown. In diagram a, the eccentric shaft A has a 
bearing at both ends; the eye-bolt B is connected to 
it at the center and is forced down when the eccen- 
tric shaft is turned, causing the two end points of 
clamp C to bear on the work. This clamping arrange- 
ment has a very rapid action with good results. The 
throw of the eccentric shaft may vary from 1/16 
inch ( 1 .5 mm) to about 1/4 inch (6 mm), depending 
upon the diameter of the shaft and the accuracy of 
the work. In cases where it is required that the 
clamp bear in the center, an arrangement such as 
that in diagram b may be used. Here the eccentric 
shaft A has a bearing in the center and eye-bolts B 
are connected to it at each end. As the eccentricity 



Ch. 10 

Fig. 10-39. Clamping with eccentric shafts. 

is the same at both ends, the eye-bolts or connecting- 
rods will be pulled down evenly when lever C is 
turned, and strap D will get an even bearing on the 
work in the center. If the force of the clamping 
stress is required to be distributed equally at differ- 
ent points on the work, a yoke may be used in com- 
bination with the eccentric clamping device. 

When it is essential to use strap D for locating pur- 
poses, guides, which are necessary for holding it in 
the required position, must be provided for the 
strap. These guiding arrangements may consist of 
rigid rods, ground and fitted into drilled and reamed 
holes in the strap, or square bars held firmly in the 
jig and fitted into square slots at the ends of the 
strap. The bars may also be round, and the slots at 
the ends of the strap half round, the principle in all 
eases remaining the same; but the more rigid the 
guiding arrangement, the more accurate the locating. 

The ordinary eccentric lever works on the same 
principle as the eccentric rods described above. 
There are a great variety of eccentric clamping de- 
vices frequently used and commercially available 
in several different models. For convenient and ef- 
ficient operation the cam or eccentric lever should 
be located so that it is actuated by a straight hori- 
zontal pull and rotated to its position of maximum 

mechanical advantage with little change in body posi- 
tion. At any position of the handle it must, however, 
have a finger clearance of at least 5/8 inch (16 mm). 
Figure 10-40a shows a cam specially intended for 
clamping finished work. It is not advisable to use 
this type of lever on rough castings, as the castings 
may vary to such a degree that the cam or eccentric 
would require too great a throw for rigid clamping. 

Fig. 10-40. A cam and an eccentric for clamping, a. For 
clamping on the part; b. For locking a leaf 

The extreme throw of the eccentric lever should, in 
general, not exceed 1/6 of the length of the radius 
of the eccentric arc, if the rise takes place during 
one-quarter of a complete turn of the lever. This 
would give an extreme throw of, say, 1/4 inch (6 
mm) for a lever having 1 1/2 inches (38 mm) radius 
of the cam or eccentric. It is obvious that as the 
eccentric cam swivels about center A, the lever be- 
ing connected to the jig with a stud orpin; face B of 
the cam, which is struck with the radius^? from the 
center C, recedes, or approaches the side of the 
work, thereby releasing it from, or clamping it 
against, the bottom, or wall, of the jig. The lever 
for the eccentric may be placed in any direction, as 
indicated by the broken and unbroken lines. An- 
other eccentric lever, is shown in diagram b. It is 
frequently used on small work, for holding down 
straps or leaves, or for pulling together two sliding 
pieces, or one sliding and one stationary part, which, 
in turn, hold the work. These sliding pieces may be 
V-blocks or some kind of jaws. The cam lever is at- 
tached to the jig body, the leaf, or the jaw by a pin 
through hole A . Hook B engages the stud or pin C, 
which is fastened in the opposite jaw, or part, to be 
clamped to the part in which this pin is fastened. 
The variety of design of eccentric cam levers is so 
great that it is impossible to show moie than the 
principles, but those examples which are shown 
embody the underlying action of all the different 

Ch. 10 



Intermediate adjustable supports require a quick- 
acting, safe, and hand-operated locking device. A 
cam-operated locking device for that purpose is 
shown in Fig. 10-41. The actual support member is 
the plunger >1. loaded by spring D. Plunger .4 has a 
tapered (conical) shank while the binder plunger B 
has a matching tapered flat. When the fixture is 
loaded, spring D keeps plunger A up against the 
work; by actuating cam C, the binder is pulled out- 
ward, and the tapered flat engages and locks the 
tapered shank on A. The double taper on both 
plunger and binder makes it impossible to press the 
plunger down, away from the work. 

Fig. 10-41. A cam-operated locking device for an inter- 
mediate support. 

The clamp is, by its design, a quick-acting device, 
This property can be combined with other design 
elements to produce devices that perform more than 
one function in one stroke, 

The quick-ac ting jig clamp, Fig. 10-42, has a han- 
dle A, threaded to fit screw B, and a cam lobe E that 
engages strap C. As handle A is turned, cam E ap- 
plies pressure to strap C. A movement of handle A 
of approximately 90 degrees, produces the clamping 
action on the work. This allows for a variation in 
the thickness of the piece to be clamped, equivalent 
to one-fourth the lead of the screw thread advance- 
ment. For example, with a 5/8-1 1 screw, a tolerance 
of plus or minus 0.01 1 inch would be allowed in the 

Fig, 10-42. A quick-acting, cam-operated strap clamp. 

work. A groove is cut in the upper surface of strap 
C, and when the strap is loose, the cam rests in this 
groove (see sectional view). About 30 degrees move- 
ment of handle A is required to cause cam E to ride 
on top of the strap, as shown by the sectional view 
at the left. 

The head of screw B has six grooves (lower right- 
hand corner), which are engaged by set-screw D to 
prevent it from turning. To adjust the lever or 
tighten the strap when parts wear, screw B is turned 
to a new position and locked in place by set-screw D 
which also serves to keep screw B from dropping out 
of the jig. It is advisable to make a positive stop for 
handle A, so that the cam will not fall into the 
groove by a 180-degree turn and so loosen the strap. 

Work can be clamped with one quick stroke, in 
the milling fixture shown in Fig. 10-43, by a cam- 
actuated clamping device. The workpiece is shown 
secured between the clamp and the form block, 
ready for the milling operation. It will be noted 
that the cam is provided with a handle having a ball 
at one end. At the completion of the cut, the han- 
dle is raised to a vertical position. This causes a 
tooth on the underside of the cam to enter a notch 
in the top of the clamp, thus moving it away from 
the form block and permitting the part to be un- 
loaded from the fixture. The ciamp is held in con- 
tact with the cam by a spring-loaded support finger 
which slides up and down on a dowel-pin. When 
another part has been placed on the form block, the 
ball is again lowered to the position shown. The 
tooth provides a positive engagement between cam 



Ch. 10 


Fig, 10-43. A cam-operated clamp for quick withdrawal. 

and clamp, moving the clamp to the left, over the 
part. The cam surfaces then force the clamp down 
on the part, holding it securely during the milling 
operation. The weight of the ball prevents the part 
from working loose due to chatter or vibration. 
Various modifications of this type of quick-acting 
clamp are commercially available. 

A bayonet-lock is a type of cam and the bayonet- 
lock type of clamping device, Fig. 10-44, is fast in 
operation and positive. The bayonet slot is milled 
in ram C. and the point of screw D (which is locked 
in place by a check-nut), slides in it. In operation, 
the part is slipped over stud A with one hand, while 
with the other hand, handle fc, attached to the ram, 
is pushed in and rotated with a single continuous 
motion. The shoulder stud A, extends into the work 
for about two-thirds of the length of the hole. This 
insures accurate location of the work and provides 
ample support against the thrust of the drill. The 
stud is flattened, as shown, to give ample drill clear- 
ance. The revolving cap B turns on a crown at the 
end of the clamping ram C and provides for a slight 

amount of float to compensate for possible varia- 
tions in the work. As clamp B remains stationary 
during the actual turning or clamping motion of 
ram C, scoring of the face of the work is avoided. 
The drill bushing F, in the jig illustrated, is per- 
manently fixed to the base. 

Toggle clamps are commercially available in so 
many types and models that they satisfy all, or al- 
most all, ordinary fixture clamping requirements. 
They occupy quite a large space, however, and the 
need for the design of a special toggle device arises 
when the clamping device has to fit within narrow 
space limits. Figure 10-45 shows a clamping device 
of this category that has been found useful on large 
work. It consists of four arms A with the ends bent 
to a right angle, and knurled, to bold the work firm- 
ly in place. These arms are pivoted on stud B, and 

Fig. 10-44. A bayonet-lock type of cam clamp. 


Fig. 10-45. A multiple toggle clamp actuated from the 
center of the fixture. 

Ch. 10 



their action is guided by blocks C, The spring han- 
dle B, is pinned to the shank of the stud, and the 
upper edge of the handle is beveled to fit rack D, 
which is fastened to the side of the base. By turning 
the handle in the direction indicated by the arrow 
the work is securely clamped. If necessary, ordinary 
straps may be added for holding the work. 

The location of drilled hold-down bolt holes 
through the steam cylinder heads for duplex piston 
pumps was often inaccurate when flat bushing plates 
of the same shape as the casting were used as jigs. 
These bushing plates were equipped with vertical 
pads around their peripheries to form nests for the 
castings. However, due to variations in the size of 
the castings, many of the workpieces would have a 
loose fit in the jigs, resulting in inaccurate location 
of the drilled holes. To overcome this difficulty, 
the drill jig seen in Fig. 10-46 was designed to ac- 
curately clamp the work at four points by means of 
a duplex toggle action. 

The two clamping arms A are mounted to slide on 
bushing plate B by means of studs C; the central 
portions of these studs passing through large holes 
in the arms to permit their free movement. Pins D 
are a loose fit in the centrally located projections on 
the clamping arms, and their lower, enlarged diame- 
ter ends are provided with flats to fit slots milled in 
the bushing plate. This permits the arms to pivot 
about these pins and to slide along the slots when 

operating handle E is rotated. Cam F, which is ro- 
tated by handle E about stud G, is connected to 
clamping arms A by links //. These links can pivot 
about the loose-fitting studs 3 joining, them to the 
clamping arms and cam. A spring-loaded latch K 
holds the cam, levers, and arms in the work-clamping 
position shown, or in the loading position, when the 
cam is rotated counterclockwise. As the cam is ro- 
tated counterclockwise, latch K will be rotated 
clockwise and links H will become aligned with each 
other. This forces clamping arms A outward, away 
from each other, so that the jig can be placed over 
workpiece X '. The cam is then turned clockwise to 
the position shown, and arms A are pulled together 
firmly to clamp the work for drilling. Ten holes 3/4 
inch in diameter, are drilled through the cylinder- 
head castings in this operation. 

The Use of Adapters 

Relatively inexpensive yet efficient fixtures are 
made by using a commercial work holder as the fix- 
ture base, then attaching special inserts to the jaws. 
Examples of lathe chucks converted to fixtures are 
shown elsewhere in the book (Chapter 9, Centra- 
lizes; Chapter 20, Miscellaneous Fixtures). The 
most common, versatile, and least expensive work 
holder suitable for conversion into a fixture is the 
machine tool vise. 

t i x / 

Fig. 10-46. A duplex toggle action clamp with four clamping points. 



Ch. 10 

The cheapest kind of milling fixture that can be 
built is a pair of detachable vise jaws, as shown in 
Fig. 1 0-47. Made of cold-rolled steel and case- 
hardened, they are inexpensive. They can be re- 
moved from the vise quickly and replaced by other 
jaws. Detachable jaws are widely used where great 
accuracy is not required, such as when cutting to 
length or milling clearance cuts. The jaws shown 
here are used for cutting off pieces from a bar of 
stock, which is pushed up against the stop and then 
cut off to the desired length. However, when the 
jaws are made with adequate precision, and the vise 
is in good condition, this type of fixture can be used 
for work with tolerances down to ±0.001 inch 
(±0.03 mm). 

S 'M 

1 1 


1 1 


- -- 



Fig. 10-47. Detachable vise jaws for holding bar stock. 

Accuracy is improved if the detachable jaws are 
fastened to the vise jaws by screws and also secured 
in position by dowel pins. The fixed jaw is the fix- 
ture base. It carries the locators for the part, and 
the machining pressure must always be directed 
against the fixed jaw. With this simple device, the 
vise has become a fixture of wide applicability. The 
possibility of using a vise with inserts should always 
be investigated in the early stage of planning for a 
small part. 

Almost all the rules of locating, and many of the 
rules of centralizing, can be applied to the design of 
vise jaw inserts. Round pans are held in V-blocks. 
Detachable jaws can be made larger than the vise 
jaws, thereby expanding the capacity of the vise and, 
at the same time, reducing its rigidity. Precision 
alignment of the two jaws is obtained by providing 
guide pins or matching tongues and slots. Insert 
faces can be machined to an angle for parts requiring 
angular cuts. Parts can be located by stops, pins, 
and nesls. Inserts can be designed to hold more 
than one part, equalizing yokes can be attached to a 
detachable vise jaw, and even ejectors can be built 
in. While most applications of the vise with fixture 
inserts are for milling operations, it can also be used 
as the base for a drill jig by the addition of a drill jig 
plate with bushings. Various types of air-operated 
vises are commercially available; naturally, they offer 

the same conversion possibilities as the hand-operated 
vise and, in addition, faster operation. The same is 
the case with cam-actuated vises. Hydraulically op- 
erated vises offer greater clamping pressures than 
any other type of vise; they are available with a 
clamping force of up to 20 tons (178 kN). 

Combinations of commercial work holders can be 
used to advantage. For example, a drill chuck, act- 
ing as a centralizer, can be fitted to be held in a 
vise, thus acting as the fixture base. A conventional 
drill chuck may be adapted to hold small work- 
pieces that might otherwise be distorted if clamped 
directly in a vise. 


Magnetic Chucks 

Magnetic chucks are available in two main types, 
as a surface plate (see Fig. 10-48) 3 usually of rectan- 
gular shape and as a face plate (for mounting on a 
rotating spindle) usually of circular shape. The body 
of the chuck can be trunnion mounted for precision 


Courtesy oft'. Thaulow 
Fig. 10-48, Magnetic chuck of the surface plate type. 

machining. For precision work the chuck has a 
reference pin mounted on one end and a matching 
flat reference surface on the base. The distance be- 
tween the tw.o reference surfaces is measured with 
gage blocks; in this way the chuck functions as a 
sine plate. The magnet poles terminate flush with 
the face of the chuck, and are separated from the 

E. Thaulow, Maskinarbejde (Copenhagen: 
Forlag, 1928) vol. I. 

G.E.C. Gad's 

Ch. 10 



chuck body by strips of nonmagnetic material (cop- 
per, brass, austenitic stainless steel, aluminum, or 
plastic) of a thickness of approximately 1/8 inch 
(3 mm). The polarity of adjacent poles alternates, 
or all individual poles have one polarity, and the sur- 
rounding face of the chuck has the opposite polarity. 
Either permanent magnets or electromagnets are 
used. Chucks with permanent magnets have less 
holding power than electromagnetic chucks, but 
have the advantage of not requiring an electric power 
supply. As long as the chuck is empty, the magnetic 
circuit is open at the chuck face. When a workpiece 
of a ferromagnetic material is placed on the chuck, 
the circuit is closed. In electromagnetic chucks the- 
circuit is permanently closed within the chuck; when 
the current is switched on, the magnets are energized, 
and the magnetic flux passes through the workpiece 
clamping the work to the chuck. Chucks with per- 
manent magnets have mechanical devices for open- 
ing and closing the magnetic circuit inside the chuck; 
the magnets may rotate or slide in and out of their 
closing position, or they are connected at their lower 

end by a sliding armature with nonmagnetic inserts 
which interrupt the circuit when the armature is 
moved to the "off" position. 

While most magnetic chucks are purchased for use 
as general-purpose work holders, it occasionally may 
be necessary to design a special magnetic chuck for 
incorporation into a fixture, and this requires esti- 
mating the holding power from some given data. 
The holding force has two components, the tensile 
holding force which prevents the part from being 
pulled off the chuck, and the shearing holding force, 
which prevents the part from sliding along the sur- 
face. The shearing holding force is essentially a fric- 
tion force and is significantly weaker than the ten- 
sile holding power. The tensile holding force de- 
pends on the strength of the magnets, the position 
of the part relative to the poles, the size of the con- 
tacting surface, and the material, configuration, and 
surface quality of the workpiece. 

The overall strength of the chuck is expressed by 
its energy consumption in watts. Specifically, the 
strength of the individual magnet depends on the 

Table 10-5. Tensile Holding Force 
AISI 1018 Steel on Electromagnetic Chuck 

Work Position 

Work Dimensions, 
Width X Height 

Relative to Poles 

Clamped on 

Holding Force 

In English Units 

Across at least 
three poles 

flat surface 


M x 1% 

1% X 1 

1% x % 

l 1 /, X % 








114 x l l / a 

1 X l'/ a 

% X l'/ s 

'/. x IV, 





1 17 

Parallel to poles, 
straddling two 

flat surface 


l'/j x 1% 

1 '/» x 1 

l'/i X % 

1% x % 








i% xi 1 /, 

1 X 1% 

% x l'/S 

% x 1% 






In Metric (SI) Units 

Across at least 
three poles 

Hat surface 


38 X 38 

38 X 25 

38 X 13 

38 X 6 

N/mm 5 

1 ,03 






38 X 38 

25 X 38 

13 X 38 

6 X 38 

N/mm 3 





Parallel to poles, 
straddling two 


flat surface 


38 X 38 

38 X 25 

38 X 13 

38 X 6 

N/mm 5 







38 x 38 

25 X 38 

13 X 38 

6 X 38 








Ch. 10 

number of ampere-turns in its coil. The significant 
material property is the magnetic permeability. Low 
carbon steel has the highest permeability of the 
common ferrous materials, and this property de- 
creases with increasing content of carbon and alloy- 
ing elements (Cr, Ni. Mo, etc.). The clamping force 
for cast iron is about 60 percent that of steel. The 
best position for the part is across the poles, cover- 
ing at least three poles. Long and narrow parts may 
have to be clamped parallel to the poles, straddling 
over two poles only. The largest holding force is 
obtained with well fitting, finish machined surfaces. 
For rough machined surfaces the holding power is 
about 75 percent and for unmachined, but fairly 
regular, surfaces it is about 40 to 50 percent of the 
best value. 

Values for AISI 1018 steel for various clamping 
conditions are listed in Table 10-5 and show that 
the height perpendicular to the chuck is highly sig- 
nificant, while the width in contact with the chuck 
has much less effect on the force per unit area. 

Compared to other clamping methods, magnetic 
clamping is relatively weak. It is widely used for 
grinding, and can be used for light milling and turn- 
ing. It is fast and convenient. Magnetic chucks are 
relatively inexpensive. Ferrous materials acquire 
remanent magnetism by this clamping method and 
must be demagnetized. 

There are several means for improving the per- 
formance of the magnetic chuck. They can all be 
described as field shapers, as they affect the shape of 
the magnetic field. Their purpose is to draw more 
magnetic flux through the work either by increasing 
the area of access for the flux or by locally increasing 
the flux density. Figure 1 0-49 shows several such 

devices. A is a block of steel called a "binder 
block." It is occasionally called a locator although 
it does not perform the functions of real locators as 
they are commonly used in fixtures. It collects flux 
from the chuck and delivers it to the work, thereby 
increasing the area of clamping as well as the clamp- 
ing force. Binder blocks arc used parallel to the 
work and also as end stops. B is a hold-down plate 
for the purpose of clamping thin workpieces C of 
nonmagnetic material; it is made of steel and collects 
some flux, sufficient to produce a clamping force on 
the work. The inserts D are flux dams; they are 
made of nonmagnetic material and divert the flux in 
such a way that the flux density is locally increased 
where greater flux penetration is required. A field 
shaper of a different design is shown in Fig. 10-50. 

-—-■■- J 

f * '1 * ~~* lw< 

Courtesy ofE. Thaulow 
Fig. 10-50, A field shaper of the V-block type for holding 
cylindrical parts. 

It consists of narrow elements with thickness and 
spacing matching the poles in the chuck. The ele- 
ments are separated either by air gaps or by nonmag- 
netic spacers, and function as pole extensions. Field 
shapers of this type come closer to the original defi- 
nition of a locator. The design shown is a V-block 
for clamping cylindrical work. 

Electrostatic Chucks 

Electrostatic chucks (see Fig. 10-51) work on the 
principle of the attraction between surfaces with 
opposite electrical charges. The chuck body consists 
of a ceramic material with an additive to make it a 
semiconductor. It is charged to about 3000 volts 
from a power supply while the workpicce "is elec- 
trically connected with the chuck frame which is 

Fig. 10-49. Section through a magnetic chuck with field 

E, Thaulow, Maskinarbejde (Copenhagen: 
Poilag, 1928} vol. 1. 

G.E.C. Gad's 

Ch. 10 










Courtesy of Electroforce, Inc., Fairfield, Conn. 
Fig, 10-51. An electrostatic chuck. 

permanently grounded. The amperage of the cur- 
rent delivered by the power supply is very low, just 
enough to make up for any leakage that may occur 
in the dielectric film. These features eliminate any 
electrical hazard. The two surfaces are insulated 
from each other by a hard resin coating on the chuck 
surface supplemented by a film of a liquid dielectric 
to prevent air from entering the interface. The same 
dielectric can also be applied to the grinding wheel 
for cooling and lubrication. A small heater main- 
tains the chuck at a temperature a few degrees above 
ambient temperature to prevent condensation of 

In accordance with Coulomb's Law the attractive 
force is proportional to the product of the charges 
and inversely proportional to the square of the dis- 
tance between the two surfaces. Since this distance 
is small, the force is quite significant, approximately 
30 psi for clean and smooth surfaces and indepen- 
dent of the thickness of the workpiece. Electro- 
static clamping can be applied to any conductive 
material; for nonconductivc materials it is sufficient 
to give them a conducting surface by spraying them 
with a quick-drying, conductive aerosol lacquer, 
which can be dissolved after the operation; or by 
mounting them on a metal foil, with an adhesive. 

Adhesive Clamping 

Clamping by means of primitive adhesives such as 
resin, varnish, and shellac have been used presum- 
ably since the beginning of the machine shop trade. 
Modern technology has developed a number of new 
adhesives for the joining of metallic surfaces. The 
use of adhesives for clamping in a fixture is deceiv- 
ingly simple but has a number of drawbacks. 

The method does not necessarily provide precision 
clamping because the adhesive film has a finite and 
significant thickness that is not always easily con- 
trolled. The curing of the assembly after adhesive 
has been applied always takes some time, it may be 
a few hours, or it may be overnight. The adhesive 
must have a matching solvent, so that it can be re- 
moved, and the removal is also a time-consuming 
process. On the other hand, the adhesive must not 
be soluble in cutting fluids. Considering all this, ad- 
hesive clamping is selected only when no other ac- 
ceptable method can be found, and is limited to 
parts with a flat clamping surface and to light ma- 
chining operations. 

Vacuum Clamping 

Clamping by vacuum is a versatile, fast, and clean 
clamping method. There are no requirements with 
respect to thickness or size of the part nor to the 
electrical or magnetic properties of the material. 
The method is applicable to flat as well as to curved 
surfaces and is not very particular with respect to 
surface quality, since the clamping area is sealed off 
all the way around. The method is distortion-free 
because the surface of the vacuum chuck is made to 
match the work surface. The theoretical maximum 
clamping force is 14.7 psi (0.1 N/mm 2 ) with full 
vacuum and is reduced in proportion if the vacuum is 
less. The available clamping pressure is sufficient for 
many machining operations on aluminum, and vacu- 
um clamping is widely used in the aircraft industry. 

Vacuum fixtures are made from cast iron or alum- 
inum. A flat clamping face is machined; a curved 
clamping face is cast to form, then ground and pol- 
ished smooth, A net of crossing grooves is formed 
or machined into the clamping face to act as a mani- 
fold for distributing the vacuum to the entire sur- 
face (see Fig. 10-52). The seal along the edge con- 
sists of a rubber hose in a groove. Vacuum chucks 
for very thin sheets do not have grooves as they 



Fig, 10-52. Design details of a vacuum chuck. 



Ch. 10 

would leave permanent markings on the plates. Small 
parts can be clamped on individual suction cups 
shown at the right in Fig. 10-52; the center plug is 
machined so that it supports the part without leav- 
ing permanent marks. 

Clamping with Low-melting Alloys 

Low-melting alloys can be used for clamping parts 
of awkward shapes, large overall dimensions, and 
thin wall sections. The use of cast materials for 
nesting purposes is described in Chapter 6 and several 
cast materials differing in melting points and pour- 
ing temperatures are listed. Most of these require 
access to the facilities of a no n ferrous foundry. For 
clamping purposes the preferred metal is Cerro 
Bend®, a bismuth alloy which melts at 158 F and 
can be handled in the machine shop without the as- 
sistance of a foundry. One technique consists in 
making a set of nesting clamps. The part serves as 
the pattern; it is coated with a parting agent, and lo- 
cated in a split mold or flask similarly coated with 
the parting agent. The metal is poured, and after 
solidification and removal from the mold, the metal 
block is machined to fit the jaws of a work holder, 
or fixture. The block is cut in two or three pieces, 
the part is removed, and the pieces of the casting are 
finally installed on the jaws, thereby completing the 
fixture, A similar technique is used for fixtures with 
one nest and an ordinary clamp. An example is 
shown in Fig. 1 0-53, s where the part is a valve hous- 



Courtesy of E. Thaulow 
Fig. 10-53. A fixture with a cast nest. 

ing with three branches. The fixture is screwed on a 
lathe spindle for machining. The nest is mounted 
on a three-position index plate by means of which 
the three branches are aligned in the spindle axis, 
one at a time. 

An entirely different technique can be used on 
large parts that are difficult to handle. A typical 
example is the fixturing of the four-legged tubular 
frame A shown in Fig. 10-54. The part is to be 
machined on the four flanges B. The fixture con- 
sists of a base C, two boxes D, two removable V- 
blocks E, and two screw jacks G. The boxes have 

E. Thaulow, Maskinarbejde (Copenhagen: G.E.C. Gad's 
Forlag, 1930) vol. II. 

Fig. 10-54. Direct clamping by means of cast metal. 

slots for the legs of the frame and serve as molds for 
the casting process. Additional equipment com- 
prises the two removable centering arbors F. The 
arbors are placed in the tubular legs; the fixture is 
placed on a surface plate and the part is set up and 
leveled by means of the screw jacks and arbors rest- 
ing in V-blocks E. The slots around the legs are 
sealed with clay, the metal is poured and when it 
has solidified the part is clamped. V-blocks«nd ar- 
bors are removed, and the loaded fixture is moved 
to the milling machine and clamped on the table 
with clamps acting directly on the cast-metal blocks. 
After machining, the fixture is set in a tray, hot 
water is poured over the metal blocks, the metal 
melts away easily and is collected in the tray, ready 
for reuse. 

Another, and more sophisticated, application is 
the clamping of gas turbine compressor blades for 
the machining of the root. The shape of the blade is 
an air- foil which, in itself, provides virtually no us- 
able clamping surface. The blade is inserted and lo- 
cated in the casting die in a regular die-casting ma- 
chine and a low-melting alloy is cast onto and 

Ch. 10 



around the blade, thereby providing a set of suitable 
clamping surfaces. 

In the aircraft industry, honeycomb in the ex- 
panded condition is clamped for machining by means 
of ice. With the honeycomb block placed on a re- 
frigerated platen, the cells are filled with water. As 
the ice is formed it supports the cell walls and 
clamps the block to the platen. 


When designing clamping devices, the fewest num- 
ber of operating screws or handles should be used as 
will accomplish the desired result. Making the screw 
with a double or triple thread is sometimes em- 
ployed to advantage in decreasing the number of 
turns necessary to release the piece. Jig lids should 
be hung on taper pins in order to compensate for 
wear in the hinge and to prevent any resultant inac- 

curacy due to lost motion in the hinge. The included 
angle of taper on hinge pins should be only one or 
two degrees. The hinge pin should be a tight fit in 
the central portion of the hinge, which is usually the 
jig body, and a bearing fit in the ears of the lid. 
All manually operated clamping screws and similar 
parts should be long enough and so located as to be 
convenient and easy to operate, and of sufficient 
size to prevent hurting the operator's hands because 
of the manipulating pressure necessary. The screws 
should be located so that they will resist the tilting 
action of the block, and dowel pins should be fairly 
close to the screws and of liberal dimensions in order 
to resist the shearing strains to which they will be 
subjected. When clamping or locating the work in 
the jig, it is essential to have the clamping pressure 
exerted in a direct line against some solid point of 
support to prevent the tendency to tilt, and the 
thrust should also come at such a point of the work 
that it will be resisted by solid metal. 





The purpose of an equalizer is, in the broadest 
sense, to distribute a single force between two or 
more separate points of action. Strictly speaking, the 
name implies an equal force distribution; but (here 
are also equalizers that distribute the force in a con- 
stant ratio not necessarily equal to one. Predom- 
inantly applied to clamping mechanisms equalizers 
are also used on locators. 

When an equalizer is applied to a clamping mecha- 
nism, the force to be distributed is the "actuating" 
force; when applied to locators it is more difficult 
to visualize the "force" because it is, in most cases, 
a reaction to one or more clamping forces. Equal- 
izers can also be applied simultaneously to a clamp 
and a locator for the purpose of either equalizing a 
clamping pressure and the locator reaction, or main- 
taining a constant ratio, not necessarily equal to one, 
between these two forces. 

Individual screws, even when threaded through the 
same fixture component, are, as a rule, not equal- 
izers. When clamping with two screws as was shown 
in Figs. 10- 1 4a and 10-2 Ob, the screws are tightened 
one at a time, and whether or not the part is clamped 
with equal pressure depends on the operator. In con- 
trast, the two pointed screws shown in Fig. 10-14b 
constitute an equalizer. When one screw is tightened 
to a certain pressure, it generates an equal reaction 
from the other screw. The same is true with the two 
eye-bolts in Fig. 10-2 i b. 

Representative Examples 

Although some equalizers are complicated, this is 
not necessarily always true. The plain double-end 
strap with a screw in the center, clamping simulta- 
neously on two parts, as shown in Fig. 10-17, is an 
equalizer, as is the three-pronged strap shown in Fig. 
10-16d. A typical equalizer is shown in Fig. 10-36; 
it is the balancing clamp in the center of the fixture. 

In Fig. 10-2 2a, an equalizing effect is obtained by 
a gripping dog. When screw E is tightened, the pres- 
sure on the work from the dog D, is increased, as is 
the pressure on the locator below A . Strap C in Fig. 
10-23a is an equalizer for the two slides B. In the 
fixture shown in Fig. 10-23b, the equalizer is rod D 
in conjunction with screw B. Actuating screw B has 
the same effect as an increase of the length of rod D 
which pushes with equal pressure on levers A and C. 

Clamp Sin Fig. 10-24 contains an equalizer, name- 
ly, the V-notch that grips over the corner of the 
work. The horizontal and vertical pressures on the 
work increase in the same ratio when screw E is 
tightened, assuming that the coefficient of friction 
between the tip of the screw and block D remains 
constant. The clamping device shown in Fig. 10-25 
is an equalizing mechanism because of its symmetri- 
ca] design. 

Many centralizers, including those shown in Chap- 
ter 9, are also equalizers. A V-block holding a cylin- 
drical part is an equalizer. The toggle-action drill 
jig shown in Fig. 10-46 contains two equalizing 
systems; one equalizes the pressure on the two ends 
of each clamping arm A (the arm A being the equal- 
izer), the other equalizes the two arms A against each 
other (the equalizer is actually the workpiece X). A 
similar effect is found in the clamping device shown 
in Fig. 10-45, where it is the workpiece itself that 
equalizes the pressure from the four arms A . 

The Floating Principle 

An analysis of these examples shows that in all of 
them the forces are transmitted through a floating 
component; that is, a component which is movable 
with at least one degree of freedom. The floating 
component is statically determinate and adjusts its 
position freely until force equilibrium is reached. In 
some cases the floating member is represented by 
the workpiece. 


Ch. 11 



The principle of action of an equalizer is often re- 
ferred to as "the floating principle." In mechanical 
language the function of an equalizer is to eliminate 
a redundancy. The required movability in the sys- 
tem is obtained either by forming the equalizer as 
a beam that can swivel around an axis, or by using 
a stiff member-such as a rod -that floats between 
two springs. Although the details may vary, most 
equalizing systems are based on one of these two 

Classification of Applications 

Basically, equalizers are used for the following 

1. To distribute an otherwise concentrated clamping 
force more evenly over the surface of a part 

2. To align clamping forces with locators 

3 . To clamp on rough surfaces 

4. To clamp on surfaces (rough or machined) of 
different heights 

5. To clamp simultaneously on a horizontal and a 
vertical surface 

6. To spread the clamping forces (and their match- 
ing locator reactions) evenly over a wide area 
to avoid distortion of a thin-walled and elastic 

7. To center a part 

8. To clamp simultaneously on more than one part 
(multiple clamping). 

The Problem Areas 

The satisfactory function of an equalizer requires 
that the design assumptions are fulfilled. This re- 
quires attention to several factors which, under ad- 
verse conditions, can restrict the movability of the 
equalizer. As applied to clamping or holding meth- 
ods, the greatest care must he exercised in order to 
make sure that the floating action is not constrained 
in any one direction, but will operate equally well, 
and with uniform pressure, on the required area. 
Frictional resistance may, in cases of this kind, be 
sufficient to cause imperfect work by reason of un- 
equal pressure on the work itself. 

Friction is invisible although it occurs wherever 
there is sliding motion. Most equalizers close with 
a slight rotation and friction forces are generated at 
the points of contact. The fixture designer must 
visualize the direction of the friction forces, estimate 
their size, and evaluate their possible effect. Signifi- 
cant friction forces occur on rough surfaces and also 
where forces are transmitted through surfaces that 

are not perpendicular to the force direction, such 
as on plungers with their ends at 45 degrees. 

When clamping action is applied to a rough surface, 
great care must be used and the amount of float 
must be so proportioned that it will take care of a 
considerable variation in the castings or forgings. 
When a great number of pieces are to be handled, 
several patterns are often used and these will be 
found to vary somewhat, thus, there are differences 
in the resultant castings. Allowance must be made 
for extreme cases of dimensional variations. 

When applied to methods of locating the work, or 
to supporting points on which it rests, the construc- 
tion must be such that it will not, by any possibility, 
cause distortion. If springs are used under support- 
ing plugs which are later to be locked in position, the 
springs must be so proportioned that they will not 
be strong enough to cause any trouble by forcing 
the piece out of its true position. Also, when sup- 
ports are placed against finished surfaces they should 
be so arranged that they will not injure them. In 
locating a piece of work from two previously ma- 
chined surfaces which are in different planes, the 
float action must be very carefully studied, to be 
certain that the contacts are positively assured, and 
no tilting of the work will result. In such cases, the 
equalizer may occupy a tilted position and it is neces- 
sary to check for the possibility of false contacts 
between the equalizer and other parts of the fixture 
and the workpiece. 

There are occasional instances which require the 
location of a piece of work from a previously ma- 
chined surface, in connection with a threaded portion 
by which it must be clamped. In a case of this kind, 
the "float" must be made so that it will take care of 
a possible lack of concentricity between the thread 
and the other finished surfaces and, at the same time, 
provide means of equalizing variations in the align- 
ment of the thread. Locking devices for floating 
members must be so arranged that the members can 
be positively locked or clamped without causing any 
change in their position. A turning action, such as 
might be caused by the end of a screw against a 
locating point, is often sufficient to throw the work 
out of its correct position. The interposing of shoes 
between screws and floating members will prevent 
any trouble of this kind. The swiveling shoe or pad 
on the end of a screw is, in itself, an equalizer applied 
to a small area, as is the spherical washer. 

The Rocker Equalizer 

The basic and most commonly used equalizer type 
is the "rocker." In principle, it is a beam, supported 



Ch. 11 

at the center and loaded at the ends. It can be a 
straight beam, such as the double-end strap, or it 
can be curved, as a yoke, when it is supported from 
above and has to reach down to the work piece. This 
form is typical and a good example is shown in 
Fig, 11-1. A is the work to be clamped, and B is the 
yoke which fits into a slot in the center of the strap, 
or clamp, C. The yoke is held by pin Z), around 
which it can swivel to adjust itself to the work. The 
amount of pressure at the two points E and F will 
be equal, even though the screws at the ends of the 
strap may not be tightened to exactly the same 
height. Pin D takes the full clamping strain, and 
should therefore be designed to be strong enough. 
The strap, which is weakened by the slot and the hole 
in the center, should be reinforced, as indicated, at 
this place. It is preferable lo have spiral springs at 
each end of the strap to prevent it from slipping 
down when the work is removed. The strap may be 
of either cast iron or machine steel, while the yoke 
should always be made of machine steel. 

A F 

Fig. 1 1-1. The rocker-type equalizing elamp. 

The plain rocker acts on two points only. By 
placing a small rocker on each end of the main 
rocker its action is expanded to four points; this 
development can be carried further and is used for 
simultaneous clamping of several parts (multiple 
clamping). Parts with large, flat areas requiring three 
clamping points can be clamped with a rocker-type 
equalizer formed as a plate, as shown in Fig, 11-2. 
The fixture is a drill jig provided with a floating 
clamp to work on a rough surface of a piston casting 
A, which has been previously machined at B. The 
body of jig G is of semi-box section and is provided 
with feet D on which it may rest, both during load- 
ing and when under the drill. A hardened and ground 
steel stud E is let into the casting at one end and 
serves as a locating point for the machined interior 
of piston B. A stud C is also provided to give the 
correct location to the wrist-pin bosses. 

As the end of the piston is of spherical shape and 
in the rough state, it is necessary to provide a means 

Fig. 1 1-2, A drill jig with a piale type equalizer. 

of clamping which will so adjust itself to the in- 
equalities of the casting that an equal pressure will 
be obtained so that there will be no tendency for 
the work to tilt, A heavy latch M is pivoted on a pin 
L, and is slotted at the other end to allow for the 
passage of a thumbscrew N which is used to clamp 
it in position. A special screw is threaded into the 
latch, and is ball-ended at />so that it has a spherical 
bearing against the floating clamp Q. The screw S 
keeps it in position, but clearance is provided to 
allow for the floating movement around the body 
of the screw. Three pins R are set 120 degrees apart, 
in the face of the floating clamp, so that a firm 
three-point bearing is assured. In order to assist in 
supporting the work under the pressure of the drill, 
two spring-pins T are provided, set in the form of a 
vee near the front end of the piston. They are en- 
cased in a screw bushing t/and are locked in position 
by means of set-screws (not shown) after they have 
been allowed to spring up against the piston casting. 
(In order to avoid confusion in the drawing, only 
one of these pins is shown and that at an angle of 
45 degrees from its a,ctual position.) 

Steel liner bushings F are provided in the body of 
the casting so that the main bushings, which are of 

Ch. 11 



the removable type, as shown at H, may not produce 
too much wear in the jig body itself. A slot is pro- 
vided in the head of the bushing so that pin K will 
prevent it from turning under the twisting action of 
the drill. It should be noted that in the construction 
of the spring-pins, which are used to help support 
the casting, the springs themselves should be very 
light so that they will not force the piston out of its 
true position, determined by the locating stud. 

Equalizing on three points can also be accom- 
plished by a system of 45-degree wedge-end plungers. 
A very rigid mechanism of this type is shown in 
Fig. 11-3. It is shown applied to a drill jig, but it is 
rigid enough to permit its use in milling or planing 
fixtures. In these cases, the clamping pins become 
rest pins and are subject to the thrust of the cut. 

Fig. 11-3. The multiple plunger type equalizer. 

Screw A thrusts against equalizing plunger B. Plunger 
B is of smaller diameter than the drilled hole and 
rests on piece C. This piece is cut from a rod of the 
same diameter as the hole and is used to afford a 
flat base for plunger B to rest on and insure full 

contact of the wedge end against plungers D and E. 
Plunger G is a duplicate of B and equalizes plungers 
F and H by means of the same mechanism. 

The Floating Screw Type Equalizer 

The second basic principle used in the design of 
equalizers is that of the floating screw. A screw and 
its nut are mounted without lengthwise fixation be- 
tween two clamps or other force-transmitting com- 
ponents. When actuated, the screw and its nut exert 
equal but opposite forces on the two adjacent com- 
ponents. A typical example, shown in Fig. 1 1-4, is 
the locating mechanism for a milling fixture in which 
two pieces are located by two plungers each, all 
operated by a single clamping operation. Lever A 
draws out plunger B and throws in sleeve C, operating 
plungers D and £*. Plungers B are smaller in dia- 
meter than plungers D and permit enough lateral 
movement to equalize plungers G through auxiliary 
plungers H. 

A fixture that shows a double application of the 
45-degree wedge-cut plungers is shown in Fig. 11-5. 
Two pieces are each clamped simultaneously, at both 
ends, all by tightening one nut. The forces are fully 
equalized regardless of irregularities on the individual 
parts. Rod A , running through the fixture, carries 
ball-and-socket washers at each end and draws the 
end clamps B and C together. These clamps are 
given a down-and-in movement against the 45-degree 
wedge ends of rods D and E. The clamping thrust 
against the rods imparts a downward movement to 
the inner clamps G and H, pulling the work down on 
the inner rest-pins. The clamps are returned by 
means of plungers K and spring J. This fixture is 
also an example of a modification of the strap with 
a wedge end,. which was shown in Figs. 10-25 and 

Double Movement Clamps 

Double movement clamps are equalizers that pro- 
duce two force components with a constant ratio, 

I-ig. 11-4. A milling fixture for two parts using the floating screw principle. 



Ch. 11 

Fig. 1 1-5. A fixture with double application of plungers. 

usually in different directions. They are used in a 
large- variety of forms and applications and offer a 
wide field for the fixture designer. A few examples 
will be shown. 

Figures 11-6 and 1 1-7 illustrate small, double- 
movement clamping mechanisms for hand milling or 
profiling use. In Fig. 11-6 the clamping pressure 
against clamp A also pulls out plunger B, raising 
plunger C and throwing the work against stop E, by 
means of plunger D. Spring plunger C is used to 
return plunger D. In Fig. 11-7 the pull-through 
clamp A on the plunger B throws the work against 
stop C, by means of plungers D and E. Figure 1 1-8 
illustrates half a fixture for milling a cylindrical con- 
cave surface on an unusual piece. The work is 
clamped against pads A and B, on previously milled 
surfaces, by means of two differentially operated 
plungers C and D. To prevent springing under cut, 
the work is backed up with the floating plunger E 
on one side and F and G on the other. The plungers 
are operated by push-rods //and J. These push-rods 
are hand operated and are clamped by bushing K 
and star knob L. 


Fig. 1 1-6. A small double movement clamping mechanism. 

Fig. 11-7. A double movement clamping mechanism with a 
floating screw and a plunger. 

Multiple Clamping 

Multiple clamping consists of clamping several parts 
in one fixture for simultaneous machining. The 
most primitive form of multiple clamping is simply 
to mount a number of identical fixture units on a 
common base. Each fixture is equipped with its 
own clamping devices and is unloaded and loaded 
separately. When the fixture is fully loaded, all 
parts are machined in one operation. The obvious 
first step to improve this simple setup is to combine 
the common fixture base and the individual fixture 
housings into one part, a casting or a weldment, but 
maintain the individual clamping devices. Besides 
its simplicity, this type of multiple clamping has 
other distinct advantages. It is cheaper, more rigid, 
and faster to set up than are separate fixtures. It 

Ch. 11 



Fig. 1 1-8. A plunger-type clamping mechanism operating on four points. 

also ensures proper clamping of nonuniform parts. 
The loading and clamping operation is faster since 
it utilizes shorter and more repetitive movements. 
The positioning of the cutter is utilized for a plurality 
of parts. The total length of travel of the cutter, and 
therefore the machining time, is also less, as there is 
a saving in idle time in the approach, end run-out, 
and traverse from one part to the next. This saving 
is obviously greater, the closer the parts are set. The 
number of idle return strokes, or travels, is reduced 
to one. 

This simple application of the multiple clamping 
principle is used to some extent in milling fixtures 
(string milling) and is widely used in fixtures for 
reciprocating machining operations; notably planing 
and surface grinding. 

The largest expense factor in this type of multiple 
clamping remains the extent of time required for 
clamping the parts individually. It would be desirable 
to be able to clamp them all simultaneously, thus 
the most drastic step to take is to hold all the parts 
in one fixture and, if possible, clamp them together 
in one operation. 

An approach to this solution is the string milling 
fixture shown in Fig. 11-9, The entire package of 
parts is clamped lengthwise by one screw; each piece 
is clamped sideways by its own individual clamping 
screw. All side screws are mounted in a swinging bar 
which can be quickly removed for unloading, clean- 
ing, and reloading of the fixture. The use of in- 
dividual screws ensures that each part is fully located 
and aligned with the side wall in the fixture. It also 
prevents any "bulging" or "buckling" of the whole 
package by the pressure from the end screw. 

BIN ft NO !Cflt*» 


Fig. 1 1-9. A string-milling fixture. 

Bulging of a Package 

Bulging or buckling of a package of parallel pieces 
may occur as a result of accidental irregularities and 
thickness variations. However, buckling may occur 
even if the pieces have flat and parallel machined 
surfaces. In that case it is an instability effect like 
the buckling of a column. The tendency to buckling 
increases with the ratio LjH (see Fig. 1 1-10). To pre- 
vent buckling, setting the end screws under a small 
angle with the horizontal, pointing downward, is 
often recommended. Another very simple and effec- 
tive means of preventing upward buckling is to place 
the end screws above the half height of the package. 
The clamping force is now an eccentric load and 
generates an uneven pressure distribution over the 
contact surface. Already having an eccentricity E = 
1/6 H, the pressure at the lower edge drops to zero, 
and with E > 1/6 H an area along the lower edge has 
zero pressure. The package is compressed along the 



Ch. 11 




— * — > 








— H 


h ■ 


Fig. 11-10. Bulging of parts in a package. 

upper surface and opens the joints along the lower 
surface. It will try to buckle downward, resulting 
in a well-controlled pressure against the fixture base. 
If no means are applied to prevent buckling, the 
number of pieces within a clamped package should 
not exceed 5. 

The principle of clamping, in a package, can be 
extended to parts that are not flat by the use of 
suitably formed spacers. An example of this is 
shown in Fig. 11-11. The spacers are mounted in 
the fixture (no loose pieces) and define the place 
where each part is to be clamped. In the present 
case, the spacers are formed as half V-blocks, which 
match the upper ends of the parts while the lower 
ends rest in full V-blocks, 

Fig. 11-11. Multiple clamping with shaped spacers. 

A somewhat unorthodox, but very versatile, design 
for multiple clamping of smalt parts is shown in 
Fig. 1 1-12. It can be used for surface grinding and 
for straddle milling, slotting, and form milling. The 
base A may be fastened to a secondary base so that 

the fixture can be easily attached to the machine 
table. The work-holding members of the fixture 
consist of two pieces B, right- and left-hand, and a 
piece C, which is dovetailed to match a correspond- 
ing dovetail in parts B. Four swinging clamp mem- 
bers D are mounted on body A and are arranged so 
that one end of each bears against a tapered portion 
of pieces B. As a result, when the work -holding unit 
is forced in the direction indicated by arrow F, 
through the action of the eccentric clamp E, clamps 
D will pivot on their bearing pins and exert a force 
against the sides of pieces B, moving them slightly 
on the dovetail on part Cand clamping them tightly 
on the work pieces. When the clamp is first being 
lined up for holding a given type of work, clamp E is 
tightened very slightly. An attempt is then made to 
pull the workpieces at the two extreme ends from 
the holder with the fingers. This is done to permit 
adjusting the clamping members D, so that the two 
pairs will have an equal clamping force. 

Fig. 11-12. Multiple clamping of a large quantity of small 

If it is easier to pull the workpiece from one end 
than from the other, the clamping pressure is not 
equal. One of the two clamping members D at the 
Light end of the holder is then removed, and a small 
amount is ground from one of its pressure areas, 
after which it is replaced and another try is made. 
By this means, repeating the test process until it 
takes a very sharp tug with the fingers to remove 
either of the end pieces when E is lightly set, the 
device is considered correctly adjusted, assuming the 
workpieces to be uniform in size. 

In most cases, operations performed on work held 
in a fixture of the type described will include only 
cuts so light that there will be little danger of the 
assembly being lifted from base A during machining. 
Should such trouble be experienced, however, it is 
only necessary to mill vees in the edges of pieces B 
where the clamping members D take their bearing, 
and grind the engaging ends of members D for a good 
line bearing in the vees. 

In the design shown, the two pieces B were origin- 
ally one, the holes being drilled first and the piece 
cut in two, afterward. Because of the way parts B 

Ch. 11 



are mounted on piece C, it is possible to separate 
them for milling any desired shape of work seats in 
these parts. Thus, when the cross section of the work 
is not symmetrical, a work seat of a certain shape 
can be milled in one member B, and a differently 
shaped work seat in the opposite member. If re- 
quired, it is also possible to have two or more sets of 
pieces 5 for different types of workplaces, which 
can be used with the same retainer piece C and the 
same base A. 

A device such as that described can be made with- 
out any recesses for workpieces and used for holding 
a number of strips of material in a line pattern for 
surface grinding or milling their edges, machining 
first one edge and then the other. Other variations 
in the design are also possible. For example, both 
pieces B can be drilled with two pairs of registering 
holes in their inner edges to receive compression 
springs, so that the clamping action will take place 
against the resistance of these springs. The device 
will then automatically open, permitting the work 
to be unloaded easily and new parts inserted when 
the clamping pressure has been removed. 

Multiple Clamping with Rockers 

The rocker principle finds many applications for 
multiple clamping. One rocker clamps two parts; 
two smaller rockers mounted on the ends of a larger 

f /Z r^O^T s i 

rocker clamp four parts. Carrying the principle one 
step further leads to an arrangement for clamping 
eight parts, all by actuating only one clamping com- 
ponent, a screw, a cam, or a hydraulic cylinder. 

An example of multiple clamping is the fixture 
which was shown in Fig. 7-22. Each of the two 
jaws actuates two rockers, and each of these clamps, 
two parts. Another fixture working on a modified 
version of the same principle is shown in Fig. 11-13. 
The clamping pressure on eight small washers is 
equalized, and the washers clamped with a down- 
and-in movement in the fixture. Rod A clamps the 
equalizers B and C, which equalize the pressure 
against D and E on one side, and F and G on the 
other. Clamps D, B, F, and G are given a downward 
pull by four plungers //, which also impart a down- 
ward pull on the inner clamps /, K, L, andM. The 
clamps are bored to receive the washers, and are re- 
turned to normal position by the spring plungers N. 


'//mmm>sA '>;/>/»;/;/)//> 


Fig. 11-13. A vise type fixture with rocker type equalizers. 

Fig. 1 1-14. The mechanics of rocker type equalizers. 

Rocker systems can be designed for equalized 
clamping of any desired number of parts, not just 
2, 4, 8, etc. In such cases, it is necessary to use 
rockers with arms of unequal lengths. Figure 11-14 
shows the arrangements for the clamping of 3, 5, and 
6 parts with equal pressure. 



Ch. 11 

Multiple Clamping with Rollers 

Multiple clamping systems can be made with cylin- 
ders (rollers) or spheres (balls) instead of rockers. 
Examples are shown in Fig. 1 1-1 S. They_ can also be 
used for the clamping of single parts with uneven 
surfaces as indicated in diagram d. However, they 
differ from rockers in their basic mechanics and 
also in some practical aspects. Rockers are beams 
and the force distribution is calculated with the equa- 
tions for the equilibrium of parallel forces. Rollers 
are solid bodies; the forces acting on a roller are not 
parallel, but converge in its axis (the center of the 
circle in the drawings) and the equilibrium condition 
for each roller is established by its free body diagram. 

Hardened steel rollers and spheres have a high load- 
carrying capacity, and a roller or ball equalizer can, 
with some skill (see Fig. 1 1-1 5d), be designed within 
less space then the equivalent rocker equalizer. The 
cost is moderate, as the rollers require no machining 
other than cylindrical grinding. There are no pins 
and bearings and no carrying structure other than 
an enclosing housing which can be made up by the 
fixture base. There are no parts that can break. To 
damage a hardened roller by direct pressure takes a 
high degree of overload and, with proper dimension- 
ing, is a very remote possibility. 

A roller-type equalizer looks, at first sight, as if it 
is statically indeterminate with a redundancy at each 
contact point on a horizontal center line. However, 

this condition changes as soon as the device is actu- 
ated. As each roller is forced downward, it attempts 
to separate the next two rollers. This eliminates 
the contact and from now on the system is stati- 
cally determinate and can be analyzed by elementary 
methods. The direction of the enclosing walls is 
significant for the mechanics of the system. The de- 
vice shown in diagram a produces two clamping 
forces, each of: P = 1/2 F; but the device shown in 
diagram b has two clamping forces, each of P = 2/3 F; 
indicating that this device is not only an equalizer, 
but is also a force multiplier. The explanation lies in 
the fact that each of the two lower rollers is forced 
into a V-block by a force parallel to one side of 
the V. 

The two devices shown in diagrams a and b are 
true equalizers. There is symmetry and the two 
clamping forces P are equal. The device shown in 
diagram c is symmetrical, and the clamping forces 
are equal, two and two, but the two inner forces P 2 
are greater than the two outer forces P, : 

P, ■ 1/3 F and P 2 = 1 (2 F 

The results quoted above assume absence of fric- 
tion. Friction, however, is always present; on a 
roller the frictional component and the actuation 
forces act on the same radius, while on a rocker the 
frictional component acts on the radius of the pivot, 
but the actuating force acts on the length of the 


p 2 r 2 'p, 

Fig. 11-15. The mechanics of roller type equalizers. 

Ch. 11 



iocker arm which is several times greater. Therefore, 
the frictional components are of greater significance 
in the roller type equalizer and should be taken 
into account in the detailed analysis with coeffi- 
cient of friction ju = 0.1 (hardened rollers and some 

Hydraulic Equalizers 

With adequate sealing of the housing the system 
of rollers can be replaced by a fluid; if pressure is 
applied to the fluid it is transmitted equally to all 
clamping points. Hydraulic pressure is excellent for 
the multiple clamping of identical parts and single 
parts of irregular contour. A general discussion of 
hydraulic fixtures is presented in Chapter 2 1 , Auto- 
matic Fixtures. A few simple applications not re- 
quiring an outside hydraulic power source will be 
given below. 

A vise can be equipped with a hydraulic jaw, as 
shown in Fig. 11-16, so that a uniform pressure is 
exerted on all of the castings, regardless of variations 
in dimensions or irregularities in their surfaces, such 
as are produced by raised part numbers or company 
names cast on the work. In the set-up shown, six 
lever castings A are clamped simultaneously for ma- 
chining both sides and the top with straddle milling 
cutters B. Hydraulic vise jaw C is drilled to provide 
oil reservoirs D, with two rows of six plungers E 
fitting into the reservoirs. The oil reservoirs are 
sealed by means of pipe plugs F, and the connecting 
oil passage G is sealed by welding plug // to the jaw 
after drilling. Snug-fitting rubber washers / are 
placed in the groove of each piston to prevent oil 

leakage. Plate K, machined to fit the tapered sides 
of the castings, is screwed to the stationary vise 
jaw /. . 

In preparing the hydraulic vise jaw for operation, 
the reservoirs are filled with oil to within 3/4 inch 
of the face of the jaw. The plungers are then care- 
fully inserted in the jaw, and the assembly is clamped 
against some parallel surface to align the tops of all 
the plungers. While clamped, one of the pipe plugs 
is loosened to permit air and excess oil to "bleed" 
from the system. This plug is then secured tightly, 
the assembly is undamped, and the hydraulic vise 
jaw is ready for service. In this case, the pressure 
is applied by closing the vise. The hydraulic system 
preferably is installed in the moving jaw so that the 
rigidity of the fixed jaw is not compromised. The 
system is completely self-contained and the vise is 
actually converted to a universal fixture. 

A similar hydraulic system comprising the oil res- 
ervoir D and the desired number of pistons E, can 
be installed in other types of fixtures, however, a 
modification is required to supply the oil pressure. 
One of the pipe plugs F is omitted; and a straight 
screw thread is provided in its place to accommodate 
an actuating screw with a piston that has a sliding fit 
in the end of the reservoir. The screw and piston 
constitute a primitive pump; as they are actuated, 
oil is displaced from the reservoir lifting the plungers. 
When the plungers have established contact with the 
part, pressure builds up and the part is clamped. 
When the screw is released, the pressure disappears, 
the plungers retract, and the part can be removed. 
The system is versatile; clamping plungers can be 
arranged in any pattern and direction as long as they 

i j 

'fV J 

n Mii'ic -HuLiLii 
V V \ 


Fig. 1 1-1 6. A vise type fixture with a hydraulic equalizer. 



Ch. 11 

communicate with the pump piston through the res- 
ervoir. High oil pressures can be produced by these 
simple means. The fixture base with the reservoir 
must be designed as a pressure vessel and dimen- 
sioned accordingly, but this usually does not present 
any problem. The installation of a pressure gage on 
large fixtures is recommended. A practical upper 
limit for the pressure in oil-hydraulic systems is 
1 0,000 pounds per square inch (69N/mm 2 ). 

The only weak spot in the system is the possibility 
of oil leakage. There is no continual replenishment 
of the oil, and even a minor leak can soon result in 
loss of the oil pressure. Should this occur during 
machining, the part may be spoiled. 

Plastic Fillings 

The leakage problem is eliminated by substituting 
a plastic medium for the oil and modifying some 
design details accordingly. Paraffin, grease, and bees- 
wax have been used in the past and are still usable, 
but are being replaced by a polyvinyl chloride resin 
(PVC). 1 It is heated to 350F for filling the fixture. 
Theoretically it has no upper limit for the applicable 

PLastiflexdl proprietary to Hastings Plastics, Inc., 1704 
Colorado Ave., Santa Monica, Cal, 90404. 

pressure, but for reasons of design safety it is recom- 
mended that the fixture not be designed to operate 
at a pressure above 15,000 psi (103N/mm 3 ). Since 
it is a plastic and not a liquid, it does not offer the 
same high degree of versatility in the design as does 
oil. The reservoirs must be fairly straight from the 
pump to the most remote clamping plunger. The 
cross-section area of the reservoir must not be less 
than I 1/2 times the plunger area. Plungers and 
pistons shall be guided in their cylinders over a 
length of at least I 1 /2 times the diameter. The fit 
shall be an intermediate between sliding and run- 
ning; a class RC2 fit is suitable; mating surfaces 
shall be machined to 32 A A roughness and then 
lapped to a true cylindrical shape; a bell mouth 
in the cylinder or a tapered end on a plunger is not 
acceptable because it would provide a tapered gap 
that could provide access for the plastic medium. 
For operating pressures above 7000 psi, the pressure 
end of plungers and pistons shall be bored out to a 
cup shape to provide a 10-degree feather edge slid- 
ing against the cylinder surface. It is recommended 
that return springs be installed on the plungers. The 
plastic medium does not provide rust protection. 
For fixtures that are out of operation for long 
periods of time, it is recommended that pistons, 
plungers, and cylinders be made of a corrosion re- 
sistant material. 



Supporting Elements 

Definition and Classification of Intermediate 

Intermediate supports are those elements in excess 
of what is basically required for geometrically com- 
plete and statically determinate definition of the 
position of the part within the fixture. They are 
used when the part does not have sufficient rigidity 
to withstand the operating forces without distortion. 

In mechanical language the intermediate supports 
are redundant, and to avoid displacing or distorting 
the workpiece, they must be compatible with the 
locators, as was explained in the text to Fig. 6-37. 
An ideal solution would be to make them of a soft 
plastic material which would yield on contact with 
the part and freeze solid after the part has been 
located. This solution is generally hypothetical (ex- 
cept in such unusual cases as clamping with a cast- 
able metal, see Fig. 10-54), but it illustrates the 
principal mechanism of intermediate supports. 

Intermediate supports are manually operated screws 
and plungers, and spring, wedge, air, and hydrauli- 
cally operated plungers. 

Method of Operation 

Manually operated devices rely on the "feel" and 
judgment of the operator for the correct application 
pressure. Screw-type supports are thumbscrews, 
wing screws, knurled-head screws, hand-knob screws, 
and torque-head screws. Common to all of them is 
the fact that they are operated directly by hand and 
usually do not permit the use of a wrench except in 
very special cases, such as the one shown in Fig. 9-11. 
Thus, the operator is not deprived of the necessary 
"feel" for the proper contact pressure which enables 
him to avoid overloading the workpiece. The ideal 
screw-type device is the torque-head screw with a 
spring-loaded clutch built into the head which sets 
an upper limit for the transmitted torque and re- 
sulting pressure. 

Springs for plungers are weak springs. Air and 
hydraulically operated plungers are designed to exert 
light pressures only. An intermediate support always 
has a locking device for rigid locking in the operating 
position. Screw-type supports have check nuts, 
while plungers are locked by means of a set-screw, a 
wedge, or a cam. Where possible, the locking device 
engages a tapered surface of the plunger so that the 
locking is positive and does not depend on friction 


Screw-type supports are frequently provided with 
a swivel head. On irregular surfaces, this serves to 
equalize the support over a larger area; on previously 
machined surfaces it prevents marring the work with 
circular scratches. This can also be achieved by 
using a copper or nylon tip on the screw. 

Commercial Components 

Many individual components for intermediate sup- 
port devices are commercially available. Several 
types of spring-loaded plungers are available as com- 
plete units (jacks) ready for mounting on the fixture 
base. Since they contain movable parts and are 
exposed to chips, they are protected by caps, shields, 
or seals in the manner shown in Fig. 8-10. 

There is no very sharp distinction in the design 
between adjustable locators and intermediate sup- 
ports. The two adjustable locators shown in Figs. 
6-41 and 6-42 can also be used as intermediate 

Screw Type Supports 

The screw type of intermediate support is the 
cheapest one to make, is rather slow to operate, and 
can only be used where there is convenient access 
for the operator's hand-in a side wall of the fixture. 
The plunger type, much more versatile, is therefore 
used extensively. 




Ch. 12 

Plunger Type Supports 

Figure 1 2-1 a shows the simplest form of plunger 
support. It is provided with a helical spring beneath 
the plunger to press it against the work. One objec- 
tion to this type of support is that the plunger A , will 
slip back under the pressure of the clamps or cutting 
tools bearing upon the work. There is also the danger 

jig and ailowed to project above the base. Plunger A 
is a sliding fit in the bushing. A cap Cis driven onto 
the end of the plunger and extends down over the 
outside of the bushing, as indicated, making the 
support dirt-proof. This device, however, as well as 
those in Fig. 12-1, is not entirely satisfactory since 
it will shift as it is tightened, although when tight- 
ened, it will remain in position. 

Fig. 12-1. Simple plunger type intermediate supports. 

of the milled flat on the plunger becoming clogged 
with dirt, so that it will not work properly. Con- 
siderable time is lost, therefore, in using this type of 
support. The method of clamping the plunger is 
also slow, as it is necessary to use a wrench in 
tightening or loosening the set-screw B. Shown in 
Fig. 12-Jb is a support which is an improvement 
over that shown in diagram a. The flat on the side 
of plunger A is milled at a slight angle instead of 
parallel with the center line, as in diagram a. This 
prevents the plunger from slipping after it is clamped. 
A pressure shoe fi-made of hardened drill rod, which 
is kept from turning by a small pin C, engaging a 
flat, milled in piece S-is used between the plunger 
A and the clamp. A wing nut D is fastened to the 
end of the screw, as shown, in order to eliminate the 
use of a wrench. 

In Fig, 12-2 another design is shown, which pre- 
sents a further improvement over those in Fig. 12-1. 
A bronze bushing B is driven into the base of the 

Fig. 12-2. A plunger type intermediate support with bush- 
ing and cap. 

Two other improved designs are shown in Fig. 1 2-3, 
diagrams a and b, 1 The end of the pressure shoe in 
diagram a is machined to fit almost half-way around 
the cylindrical surface of the plunger. This increases 
the pressure-transmitting surface and permits the use 
of a much larger locking pressure, resulting in a rigid 
locking of the plunger. The device in diagram b is 
an example of the use of the 45-degree wedge-end 
plunger which produces a much smaller side load 
on the plunger than the pressure shoe B, in Figs. 
12-lb and 12-2. 

Courtesy of E, Thauhw 
Fig. 12-3. Improved plunger type intermediate supports. 

An intermediate support of the wedge-operated 
plunger type is shown in Fig. 12-4. It represents a 
modification of, and an improvement upon, the 
adjustable locator shown in Fig. 6-40. The design 
of the device is low and is for use in cases where the 
plunger is located in the middle part of a fixture 

E. Thaulow, Maskinarbejde (Copenhagen: G.E.C. Gad's 
Foilag, 1930) vol. II. 

Ch. 12 



Fig. 12-4. A wedge-operated, plunger type intermediate 

and the actuating means must be accommodated in 
the fixture base. The plunger is adjusted up and 
down by the horizontal movement of the wedge. 
In Fig. 6^10 the wedge is located in a groove in the 
bottom of the fixture base and receives its support 
from the machine tool table. In Fig. 12-4 the wedge 
and its actuating rod are housed in a bore within the 
fixture base, resulting in a fully self-contained device, 
a stronger and more rigid fixture, and a more accur- 
ate operation of the wedge. The wedge is locked 
by clamping the actuating rod in the casting B, by 
means of a knurled nut which also serves as the 
handle for actuating the wedge. The wedge is sup- 
ported on the lower edges of two holes in bushing^ . 
For easy alignment, bushing A is made with a sliding 
fit and is secured in its proper position by screw C 
and dowel pin D. Full alignment is facilitated by 
the hinged connection between the wedge and the 
actuating rod. 

Equalizers and Floating Supports 

Intermediate supports can be combined with equa- 
lizers. An equalizer can carry two or more inter- 
mediate supports, or an intermediate support can 
carry an equalizer. Equalizers and "the floating 
principle" are used mainly where a plurality of in- 
termediate supports are applied to a thin plate or 
rim. In the lathe fixture shown in Fig. 9-11 three 
intermediate supports are formed by shell bushings 
K with hook bolts F acting together as the jaws of a 
small vise. Each pair of jaws can float separately 
and adjust its position to the rim of the casting A 
after the casting has been located (centered) on the 
cone locator B. Finally, the three pairs of jaws are 

clamped separately in position without distorting 
the rim. 

The fixture shown in Fig. 12-5 is for a combined 
lathe operation and.includes three intermediate sup- 
ports mounted on a common floating ring. The 
work A is to be bored, shouldered, and faced, com- 
plete in one setting. Because of its length, it was 
considered necessary to provide additional support- 
ing points besides the jaw surfaces. A set of special 
jaws B was keyed to the sub-jaws in the table at D, 
with each special jaw shouldered at C to support 
the work. The brackets E are tongued at F to fit 
the special jaws and are secured by screws G. These 
brackets act as a support for the steel floating ring 
M in which the three spring-pins J are placed. 

Fig, 12-5. Intermediate supports mounted in a floating 



Ch. 12 

Elongated holes at points N allow for the required 
floating action, as the ring is clamped by collar-head 
screws. Each bracket on which the ring rests is 
provided with a shelf// which is offset slightly from 
center to allow the necessary width for the screws. 
In using the device, screws L and N are loosened, and 
the work is placed in the jaws, which are then tight- 
ened, while the ring floats sufficiently to allow for 
variations. It will be noted that the pins, being 
spring-controlled, adapt themselves to the casting 
and are locked there by screws L, after which the 
ring itself is clamped by the collar-head screws N. 

Although the floating action of this device was 
satisfactory, the driving or gripping power was found 
insufficient to hold the work securely, thus it was 
necessary to replace the spring-pins with square-head 
set-screws, cup-pointed, and the ring was then tapped 
out to receive them. The ring was also allowed to 
float while these screws were lightly set up on the 
work, after which the clamping screws N were tight- 
ened. After this change in construction, the action 
of the mechanism was much improved, and the 
driving power was found to be sufficient. 

A different case of a floating intermediate support 
is shown in Fig. 12-6, also used for gripping a thin 
wall of a workpiece. The work has a narrow flange 


Fig. 12-6. A floating intermediate support applied to a 
thin wall. 

(at x) and clamp A has a hook that grips over the 
flange. The rim of the part is clamped between 
point x and the dog C when wing nut B is actuated. 
The left end of the clamp has an elongated hole that 
permits floating so that each clamp adjusts itself 
to irregularities in the shape of the part. 



Cutter Guides 

Definitions and Principal Types 

Cutter guides (setting gages, setting blocks, set-up 
gages) arc used for correctly positioning the cutting 
tool relative to the work and thereby eliminating 
the necessity of taking trial cuts, measuring the part, 
resetting the cutter, etc. The cutter guides are 
usually small, flat or profiled blocks, or complete 
templates, and are permanently, or semipermanently, 
mounted on the fixture. When the cutter is cor- 
rectly set, relative to the fixture, and the work is 

correctly located within the fixture, then the cutter 
is also correctly positioned relative to the work. 
Cutter guides are used extensively on fixtures for: 
milling, planing, and shaping operations. Cutter 
guides for drilling and boring operations take the 
form of bushings and are described in Chapter 14. 
On lathe fixtures, flat cutter guides are used for 
positioning the cutter for facing cuts, while curved 
cutter guides with special curved feeler gages are 
needed for positioning the cutter for cylindrical turn- 
ing operations. Turning cuts taken with cutters 

m&& '0\ «3, 


r-tii ir£3 Hi 

Fig. 13-1. Principal typesof cutter guides: F, feeler gage; GB, gage block; SF, special feeler gage. a. Setting depth of cut for 
a single-point cutting tool and a milling cutter, b. Setting depth and side position of a milling cutter, c. Setting 
depth and side position of a milling cutter with a cutter guide made for two different positions, d. Setting lathe 
tool for facing and cylindrical turning, e. Reversible guide for two side positions, f. Reversible guide for four 
dimensions, g. The use of a gage block in combination with a cutter guide. 




Ch. 13 

mounted on a turret do not require a cutter guide 
as the tools are already preset; other turning opera- 
tions may require a higher accuracy than that ob- 
tained by using a cutter guide so that diameter 
measurements must then be taken. Fixtures for 
grinding operations do not employ cutter guides, but 
may be equipped with pre-positioned grinding- wheel 

Tooling blocks with preset cutters do not, as a 
rule, require cutler guides. Cutter guides are always 
made of wear-resistant material, usually of hardened 
tool steel, but sometimes of tungsten carbide. They 
are mounted by means of screws and secured in 
position by dowel pins. They can also be manually 
held in position against a reference surface on the 
fixture. The reference surface of a cutter guide is 
usually set back a certain distance from the path of 
the cutter and the cutter is positioned against a 
feeler gage, or a gage block, placed on the reference 
surface. In this way the cutter does not have to 
come into contact with the reference surface which 
is a hardened precision surface, and both the refer- 
ence surface and the cutting edge are protected 
against accidental damage as well as excessive wear. 
It is a recommended practice to standardize the set- 
back distance, which should be not iess than 1/32 
inch (0.8 mm). However, when the setback is not 
standardized, the required feeler gage dimension must 
be clearly marked at a place close to the reference 
surface. The principal types of cutter guides for 
single and multiple operations are shown in Fig. 13-1. 



I a stop - MITH , S 


Courtesy of Cincinnati Milaeron Inc. 
Fig. 1 3-2. Manually operated milling fixture with two grip- 
ping dogs and one fixed locator, for locating and 
clamping the part; and with one centrally located 
cutter guide for setting the depth. 

tween the two cutters. Consequently, the cutters 
can be positioned by means of one 0.0625-inch 
( 1.50-mm->thick feeler gage. 

Another, more complicated, case involving the si- 
multaneous positioning of three sets of milling cut- 
ters is shown in Figs. 13-4 and 13-5. Figure 13-4 
shows the part and the fixture. The part is a pres- 
sure plate with three sets of lugs spaced 1 20 degrees 
apart. Each lug is milled on two sides, and the gang 
of cutters consists of one slitting cutler and two 
half side mills (Fig, 13-5), The operation is per- 

Cutter Guides for Milling Fixtures 

An example of the use of a cutter guide in a mill- 
ing fixture is shown in Fig. 1 3-2. The operation is 
the simultaneous milling of a contour consisting of 
seven parallel surfaces, by one gang of milling cutters. 
The side positioning is not critical and is done in the 
setup by direct measuring between the fixture and 
the milling cutters. The positioning for depth of cut 
must be repeated after each cutter grinding, and 
one cutter guide is provided, located in the center 
line of the fixture. In this case, the cutter guide is in 
the form of a button and is mounted by a press fit 
in the fixture base. 

A cutter guide (set-up gage) for side positioning is 
shown in Fig. 13-3. The operation is the straddle 
milling of the flanged edges of a pinion bearing, 
shown in chain-dotted lines. The part is located on a 
cylindrical locator and is clamped between the lo- 
cator and the modified movable jaw of a vise. This 
jaw also carries the cutter guide which is made 1/8 
inch (3 mm) less in width than the distance be- 






— •— 1 I FEELER 

Qj \ 


(T) r °« 5 FEEL " {— j 

< — UhrM- 





Courtesy of Cincinnati Milaeron Inc. 
Fig. 1 3-3. A modified milling-machine vise for a straddle- 
milling operation with one centrally located cutter 
guide for setting the side position of (he cutters. 

Ch. 13 



Courtesy of Cincinnati Milacron Inc. 
Fig. 13-4. Milling fixture for a special three-spindle milling 
machine for simultaneous milling of three sets of 
lugs on a pressure plate (outline of the part in- 

formed on a special manufacturing type milling ma- 
chine with three spindles operating simultaneously 
on {he three lugs. The part is located on a circular 
locator (the supporting stud) designed for jam-free 
entering and is clamped on its periphery by three 
angular clamps simultaneously actuated by a floating 
cam for equalized clamping. 

A cutter guide in the form of a stepped gaging lug 
is provided for each of the three places and the three 
cutter guides are mounted on a common dummy 
plate of the same dimensions as the part itself. The 
dummy plate is clamped in the fixture and the cut- 
ters are positioned, one set at a time. After removal 
of the dummy plate, the set-up is ready for produc- 
tion. With the dimensions shown, the mean thick- 
ness of a lug is 0.4965 inch (12.61 mm), leaving a 
total clearance of approximately 1/16 inch (approx. 
1.5 mm) between cutters and gaging lug. The thick- 
ness of the feeler gage is selected to be 0.0625 inch 
(1.5 mm), and the reference surface on the lug must, 
therefore, be offset from 0,2505 inch (6.363 mm) to 1 
0.2520 inch (6.501 mm), relative to the center line 
of the plate. 

Courtesy of Cincinnati Milacron Inc. 

Fig. 13-5. A dummy plate simulating the part shown in 

Fig. 13-4, provided with three cutter guides 

(gaging lugs) for positioning three sets of milling 


A cutter guide for the setting of a planer tool is 
shown in Fig. 13-6. 1 The part is a lathe bed, and the 
cutter guide is formed as a template for the com- 
plete contour of the ways on top of the lathe bed. 

Courtesy of k\ Tkaulow 
Fig. 13-6, A cutter guide for setting a planer tool. 

'E, Thaulow, Maskinarbejde (Copenhagen: G.E.C. Gad's 
Forlag, 1930) vol. II. 



Drill Bushings 

Definitions, Action, and Classifications 

Bushings are used as cutter guides for drills, 
counterbores, reamers, and other cutting tools in 
the same category. They serve a triple purpose of 
positioning, guiding, and supporting the cutting tool. 

The twist drill is a tool that is not well-suited for 
precision work. Its leading point, the chisel edge, 
has a rake angle of approximately minus 60 degrees. 
With a negative rake angle of this magnitude, metal 
removal is effected more by squeezing it away than 
by cutting. The chisel edge, therefore, is constantly 
exposed to a large axial cutting force. The angle 
between the two main cutting edges is 118 degrees 
(nominally, sometimes slightly less). An angle of 
this magnitude is not very effective in centering the 
tool. The shank of the drill is relieved with a slight 
"back taper," so that there is no contact (and 
therefore no support) between the body of the drill 
and the walls in the hole in back of the two corners 
of the lips. The circular cross section of the drill is 
reduced by the two large flutes, leaving a cross 
section similar to that of an I-beam, and consequent- 
ly, each cross section of the drill has one direction of 
low rigidity. In addition, the grinding of a twist 
drill is, for economical reasons, a very fast operation 
and, therefore, not a high-precision process. When 
starting the cut, the drill's chisel edge has a tend* 
ency to "walk" on the surface before it starts to 
bite. Also, during cutting, the drill is sensitive to 
local variations in the hardness of the metal and 
may react by running out to the side of the softer 
metal. The result is that without taking proper 
precautions, holes drilled with a twist drill, are over- 
size, out of round, displaced, out of alignment, and 
not even straight. These deficiencies are greatly re- 
duced and the quality of the work significantly 
improved by the application of a drill bushing so 
located that it provides positioning, guidance, and 
support to the drill at a point as close as possible to 
the surface of the work. 

Bushings are constantly subject to wear when in 
use and must be made of wear-resistant material. 
No single fixture component offers such a large 
variety of types, shapes, and sizes, as bushings. USA 
Standard bushings arc available in more than 50,000 
different configurations, counting all differences in 
types, sizes, and individual dimensions. In compar- 
ison, the cross-reference conversion tables of other 
fixture components include approximately 14,000 
items. Drill bushings are precision parts. They are 
commercially available at prices thai are a fraction 
of what it would cost to make them individually. 
This fact has greatly contributed to the reduction in 
the cost of fabricating drill jigs. 

Bushings can be classified as stationary press fit 
bushings and renewable (loose) bushings. The term 
"fixed bushings" for press fit bushings is not rec- 
ommended, because it is used with a different 
meaning in the text of the USA Standard. By shape 
they can be classified as headless and head type 
bushings; by use, as liner and renewable wearing 
bushings. By basic design they are classified as 
conventional and special bushings. Conventional 
bushings are classified as standard and nonstandard 
bushings, depending on individual dimensions. 

Standard bushings satisfy the majority, but not 
all, of the fixture designer's needs. In some cases a 
standard bushing can be modified to suit special 
requirements; in other cases, it may be necessary 
for the fixture designer to design a nonstandard 
bushing. Empirical rules for such designs will be 
presented. Many of the statements that will be 
made in the following concerning standard bushings 
are actually of a general nature and apply also to 
nonstandard bushings. 

Standard Bushings 

Illustrations, data, and other information about 
USA Standard bushings have been extracted from 


Ch. 14 



American Standard Jig Bushings( ANSI B 94.33-1 962, 
redesignation of B5.6-1962), with the permission of 
the publisher, The American Society of Mechanical 
Engineers, United Engineering Center, 345 E, 47th 
St., New York, K.Y. 10017. A condensed but 
comprehensive extract from this standard is found 
in Machinery '& Handbook, ' 

Standardized bushings are shown in Fig, J4-1 and 
the six basic types are shown in Fig. 14-2. They 
comprise press-fit and renewable bushings. Press-fit 
bushings are either liner bushings or press-fit wear- 
ing bushings, however, for the same diameter, liner 
bushings are shorter. Liner bushings are provided 
with and without heads and are permanently in- 
stalled in a jig to receive the renewable wearing 
bushings. They are sometimes called "master bush- 
ings." Press-fit wearing bushings to guide the tool 
are for installation directly in the jig without the 
use of a liner and are employed principally where 
the bushings are used for short production runs and 
will not require replacement. They are also intended 
for use where the closeness of the center distance of 
holes will not permit the installation of liners and 
renewable bushings. Press-fit wearing bushings are 
made in two types, with heads and without. 

Renewable wearing bushings to guide the tool are 
for use in liners which, in turn, are installed in the 
jig. They are used where the bushing wiy wear out 
or become obsolete before the jig, or where several 
bushings are to be interchangeable in one hole. 
Renewable wearing bushings are divided into two 
classes, "fixed" and "slip." 

Fixed renewable bushings are installed in the liner 
with the intention of leaving them in place until 
they are worn out, Slip renewable bushings are 
interchangeable in a given size of liner and, to 
facilitate insertion or removal, they are usually made 
with a knurled head. They are most frequently used 
where two or more operations requiring different 
inside diameters are performed in a single jig, such 
as where drilling is followed by reaming, tapping, 
spot facing, counterboring, or some other secondary 

All standardized outside diameters are in multiples 
of 1/64(0.0156) inch and all lengths are in multiples 
of 1/16 (0,0625) inch. The standard lengths of the 
press-fit portion of these jig bushings are based on 
standardized or uniform jig plate thicknesses of 
5/16, 3/8, 1/2, 3/4, I, 1 3/8, and 1 3/4 inches. Drill 
bushings in metric sizes are readily available from 

major bushing manufacturers, although not at this 
time considered shelf items. 

Standard jig bushings are designated by the follow- 
ing system: 

Inside Diameter: 

The inside diameter of the hole is specified by 
decimal, letter, number, or fraction. 

Type Bushing: 

The type of bushing is specified by letters: 

S for Slip Renewable 

F for Fixed Renewable 

L for Headless Liner 

HL for Head Liner 

P for Headless Press Fit 

H for Head Press Fit 

Body Diameter: 

The body diameter is specified in multiples of 
1/64 inch. For example, a 1/2-inch body diameter 

= 32/64= 32. 

Body Length: 

The effective or body length is specified in 
multiples of 1/16 inch. For example, a 1/2-inch 
length = 8/16= 8. 

Unfin ish ed Bushings: 
All bushings with grinding stock on the body 
diameter are designated by the tetter U following 
the number. 


.5000- S-48- 16 

Inside Diameter Hole Size: 

1 . Decimal 

2. Letter 

3. Number 

4. Fractional 

Type Bushing: 

1. S - Slip Renewable — 

2. F - Fixed Renewable 
L - Headless Liner 
HL - Head Liner 

P — Headless Press Fit 
H - Head Press Fit 

c. Body Diameter: 3/4 inch = 48/64 = 48 J 

d. Body Length: I inch = 16/16= 16 

'Eric Oberg and t'.D. Jones, Machinery's Handbook (New 
York: Industrial Press Inc., 1971) 19th ed., pp. 1884-1894. 

Tolerance on fractional dimensions where not 
otherwise specified shall be plus or minus 0.01 




Ch. 14 










Courtesy ofASME 
Fig, 14-1, Standardized types of drill bushings. 

The maximum and minimum values of hole size, 
A, shall be as follows: 

Nominal Size of Hole 

Maximum, inches 

Minimum, inches 

Above to 1/4 in., 


Nominal + 0.0004 

Nominal + 0.0001 

Above 1/4 to 3/4 in., 


Nominal + 0.0005 

Nominal +0.0001 

Above 3/4 to WS ins., 


Nominal + 0,0006 

Nominal + 0.0002 

Above 1V4 inches 

Nominal + 0.0007 

Nominal + 0.0003 

Diameter A must be concentric to diameter B within 
0.0005 T.l.V. on finish ground bushings. The body 
diameter B, for unfinished bushings, is larger than 
the nominal diameter fa order to provide grinding 
stock for fitting to jig plate holes. The grinding 
allowance is: 

0.005 to 0.010 inch for sizes S/32, 13/64, and 1/4 

0.010 to 0.015 inch for sizes 5/16 and 13/32 inch, 

0.015 to 0.020 inch for sizes 1/2 inch, and up. 

plus 1/32 inch. The included angle at the bottom 
of the counterbore is 1 18 degrees, plus or minus 2 
degrees. The, depth of the counterbore ranges from 
1/4 inch for the smallest bushings to 5/8 inch for the 
largest bushings and is adjusted to provide adequate 
drill bearing length. 

Bushings are straight both inside and out. Older 
literature shows jig bushings that are tapered on the 
outside but this can be considered obsolete. The 
upper corners, on the inside, are given a liberal 
radius (radius D fa Fig. 14-2) to allow the drill to 
enter the hole easily, while the outer corners, at 
the lower end of the outside, are chamfered so that 
it is easier to drive the bushing into the hole when 
making the jig, and also to prevent the sharp corner 
on the bushing from cutting the metal in the hole 
into which the bushing is driven. In addition, it is 
recommended (but not standard practice) to relieve 
the outer surface at the lower end by 0.00 1 to 
0.002 inch on the diameter, on a length equal 
to 1 1/2 to 2 times the length of the chamfer. 

Bearing length for the drill within the bushing 
ideally should be a function of the drill diameter. 
A bearing length that is too short causes premature 
wear, while one that is too long causes excessive 
friction (the twist drill is never a precision tool!). 
If there are no overriding conditions, a bearing 
length of 2 times the drill diameter can be taken 
as a good, workable average. There are, however, 
other considerations, such as the need for sufficient 
length of press seat in the jig wall and for adequate 
Thickness of the bushing head. An analysis of the 
standard tables shows a wide range of bearing length 
for each drill size. The ratio of bearing length (taken 
as the average of the table values for each drill size) 
to drill diameter varies from 7 for 1 /4-inch-diameter 
drills to about 1 1/4 for 1 -inch-diameter drills and 
down to 0.8 for large drills about 2 inches in 

Mounting of Bushings— Press Fit Bushings 

The length C is the overall length for the headless 
type and length underhead for the head type. 

When renewable wearing bushings are used with 
liner bushings of the head type, the length under 
the head will still be equal to the thickness of the 
jig plate, since the head of the liner bushing will be 
countersunk into the jig plate. All bushings ranging 
from 0.0135 through 0.3125 inch will be counter- 
bored to provide for lubrication and chip clearance. 
However, bushings without counterbore are optional 
and are furnished upon request. The size of the 
counterbore is the inside diameter of the bushing 

Stationary bushings of all types are mounted, as a 
general rule, by a press fit. In any press fit installa- 
tion, metal is displaced and distortion occurs in the 
bushing and in the jig plate. It is recommended to 
always use the minimum interference necessary to 
safely retain the bushing in the jig plate. A diametral 
interference of 0.0005 to 0.0008 inch is adequate 
for the installation of headless press fit bushings and 
liners with 3/4-inch to 1-inch OD. For sizes from 
1/2-inch to 3/4-inch OD, the use of 0.0003- to 
0.000 5 -inch interferences is recommended; for sizes 
below 1/2-inch OD, an interference of 0.0002 to 

Ch. 14 








4 ■ 










— $ 



Fig. 14-2. The six basic types of drill bushings. 

^~~ T 

Courtesy oj ASME 



Ch. 14 

0.0003 inch should be used. These values are for 
jig plates made of cast iron and low carbon steel. 

Example -When a bushing of 1/2-inch ID, 3/4- inch 
OD, and 3/4-inch length is pressed into a 3/4 inch 
thick jig plate with an 0.0006-inch interference fit, 
the !D of the bushing will be reduced by approxi- 
mately 0.0002 inch. The bore of the bushing is man- 
ufactured with a plus tolerance of 0.0001 to 0.000S 
inch relative to the nominal drill size, and the com- 
pression of the bushing quoted above reduces the 
bushing diameter to almost exactly the nominal drill 
diameter. At the same time, the distortion of the 
jig plate is held to a negligible amount. 

Head type press fit bushings and liners can be 
installed with 0.0003- to 0.0005-inch interference 
because the contact between the head and the 
surface of the drill plate provides additional support 
and rigidity in the assembly. Head type bushings are 
particularly recommended for installation in relative- 
ly thin jig plates where a headless bushing would 
require an excessive interference fit for adequate 
retention. The hole in the jig plate must be finished 
by means of a reamer, a jig borer, or a jig grinder, 
never with a twist drill. A twist drill cannot be 
relied upon to produce a hole of the required 
tolerance and roundness. 

The OD tolerances on bushings were established 
for the purpose of matching holes finished with a 
chucking reamer. Reamers are commercially supplied 
with a plus tolerance and produce holes slightly 
larger than their own physical diameter. 

Example -A 3/4-inch chucking reamer can be ex- 
pected to measure 0.7505 inch when in good condi- 
tion and to produce a hole very close to 0.7510 
inch in diameteT. With the standard tolerances of 
0.7515 to 0,7518 inch on a 3/4-inch OD headless 
press-fit bushing, this leaves an interference of 
approximately 0.0005 to 0,0008 inch. 

Particular precaution must be taken if two bush- 
ings are close together; the thin bridge of metal 
between them will yield excessively, and the bush- 
ings will "walk." In such cases, It is recommended 
either to use bushings that are so large that the 
holes will blend together (and flatten the bushings 
on one side so that they can contact each other), 
or to make a special insert with two bushing holes. 

A jig borer or jig grinder can hold tolerances to 
0.0001 inch economically. A psychological peculiar- 
ity of some jig borer operators is that they some- 
times develop the habit of working to the lower 
limit of the tolerances, resulting in interferences that 
are systematically on the high side. 

The recommended method of installing press fit 
bushings is with an arbor press, but if the jig is too 
large, alternate methods must be considered such as 
pulling the bushing into place with a bolt. The 
bushing is first carefully started into its mounting 
hole; drilled pressure plates are placed both on the 
free end of the bushing and on the far side of the 
jig plate; a bolt is inserted, a nut is screwed on, and 
as the nut is gradually tightened, the bushing slides 
safely into place. When the bushing is too small to 
permit the use of this procedure, it can be carefully 
driven into place by means of a hammer. The 
hammer must never strike the bushing itself; a 
punch of soft metal such as aluminum, lead, or 
brass must be used as a cushion for each hammer 

Before installing the bushing, the contacting sur- 
faces must be carefully cleaned and lubricated. 
White lead is suitable for this and has the advantage 
of facilitating later removal of the bushing if re- 
placement is needed. Recommended also is the 
handstonjng of the leading ends of the bushing, the 
chamfer edge, and the edge of the relief. 

Excessive interference may reduce the diameter of 
the bore to below a working clearance, which can 
cause tool seizure or prevent the insertion of a 
renewable bushing. An undersize bore must be 
relapped, and the operation must be performed 
with great care to prevent the bushing from becom- 
ing "bell mouthed." Another method is to forcefully 
run a drill up and down in the bushing; this will 
either destroy the bushing or open it up; at any rate, 
it certainly damages the drill and is not a recom- 
mended procedure. 

Distortion of a bushing as a result of excessive 
interference is essentially limited to the lower portion 
of the bushing. This is due to an "ironing" effect on 
the metal in the jig plate whereby small irregularities 
in the surface are squeezed down and the effective 
interference is reduced accordingly. Thus, a point 
in the surface at the top has been in contact with 
and "ironed" by the full length of the bushing, while 
points further down suffer less and less "ironing" 
effect due to the shorter length of bushing to which 
they are exposed. 

Modern technology has provided means for elim- 
inating problems associated with the use of an 
interference fit. An adhesive 2 is now available that 
will bond a steel bushing in a clearance hole in a 
metal plate as securely as an interference fit. The 
surfaces are carefully cleaned, adhesive is applied, 

2 American Loetite® bonding agent, proprietary to American 
Drill Bushing Co. 

Ch. 14 



the bushing is installed, and the assembly is left to 
cure for about four hours at room temperature. The 
curing time can be reduced to 15 or 20 minutes by 
the application of heat. Maximum temperature 
recommended is 2 5 OF and the curing process can 
be performed in an oven or by locally applying heat 
from a heat lamp or other mild heat source. 
For extremely precise location in the jig plate, the 
clearance should be 0.0001 to 0.0003 inch, resulting 
in a location accuracy of approximately 0.0001 inch. 
For average conditions, the clearance can be 0.0003 
to 0.001 inch, and for noncritical conditions it is 
even possible to go to 0. 003-inch clearance with 
good retention of the bushing. 

Installation of Bushings— Renewable Bushings 

When removable bushings are used, they should 
never be placed directly in the jig body, "unless the 
jig is to be used only a few times. The hole, 
however, should always be provided with a lining 
that is made in the form shown in Fig. 14-3 a, If the 
hole bored in the jig body receives a loose or re- 
movable bushing directly, its insertion and removal 
(if the jig is frequently used) would soon wear the 
walls of the hole, and in a short time, either the jig 

Fig. 14-3. Mounting of liner bushings. 

would have to be replaced, or at least the hole would 
have to be rebored and a new removable bushing 
made to fit the now larger-sized hole. In order to 
overcome this, the hole in the jig body is bored 
wide enough to receive a lining bushing, which is 
driven into place. This lining bushing, in turn, 
receives the loose bushing, the outside diameter of 
which closely fits the inside diameter of the lining 
bushing, as shown in Fig, 14-3b in which A is the jig 
body, B the lining bushing, and C the loose bushing. 
When no removable bushings are required, the 
lining bushing itself becomes the drill bushing or 
reamer bushing, and the inside diameter of the 
lining bushing will then fit the cutting tool used. 
The bushing may project, as shown in Fig. 14-3c, to 
provide the drill with the proper guidance and 
support close to the work. If the jig plate is thin, 

it can be locally increased in thickness by means of 
a boss, as shown. 

Head type press-fit bushings are used to prevent 
the bushing from being pushed through the jig 
plate by the cutting tool. The most important 
application is to take the thrust of a stop-collar, 
which is clamped on the drill, to allow it to go 
down to a certain depth, as shown in Fig. 14-4a ill 
which C is the stop-collar, D the wall of the jig, and 
B the press-fit bushing; F is the work. If the work 
to be drilled is located against a finished seat, or 
boss, on the wall of the jig, and the wall is not 
thick enough to take a bushing of standard length, 
then it is common practice to make a bushing having 
a long head, as shown in Fig, 14-4b. The length A, 
of the head, can be extended as far as is necessary to 
get the proper bearing. As the bushing is driven into 
place and the shoulder of the head bears against the 
finished surface of a boss on the jig, it will give the 
cutting too! a bearing almost as rigid as if the jig 
metal surrounded the bushing all the way up. 

Removable bushings (Fig. 14-4c) are frequently 
used for work which must be drilled, reamed, and 
tapped. Each of the cutting tools has its own bush- 
ing, and all these bushings have the same outside 
diameter so that they fit into the liner. There is a 
sliding fit so that they can be gently pressed into the 
liner by hand. These bushings are termed "slip 
bushings." They are also used when different parts 
of the same hole are to be drilled out to different 
diameters, when the upper portion of the hole is 
counterbored, or when a lug has to be faced off. A 
slip bushing belongs to one tool only, and must not 
be interchanged. Slip bushings for drills and reamers 
are nearly the same size and cannot be recognized by 
sight alone. They must, therefore, be identified by 
conspicuous markings, usually the letter R, on a 
reamer bushing. 

a be 

Fig. 14-4. Head type bushings. 

The outline of the bushing shown in Fig. 14-4c is 
in accordance with the standardized designs shown 
in Fig. 14-2. Attention is drawn to the groove E, 



Ch. 14 

that is cut immediately under the head B, and is 
also shown in Fig. 14-2. Although its dimensions are 
not standardized, the groove is important because 
it provides clearance for the grinding wheel, A 
width of 0.080 inch is suitable. Occasionally, a 
bushing having a large outside diameter is required 
as, for example, when a large counterbore must be 
used in a small hole, which makes it necessary to 
have a large opening in the jig body. If several 
operations with tools of different diameters are re- 
quired, then all the slip bushings for these tools must 
be made with the same large outside diameter. 

Slip bushings as well as other renewable bushings 
must be secured (locked) in place; otherwise they 
may rotate with the tool, wear rapidly on the out- 
side, or be forced out by the chips. A number of 
design details for such locking devices have been 
developed. Their common requirements are that 
they must be safe, effective, simple, foolproof, 
chip-proof, and, preferably, inexpensive. 

A very strong, safe, and effective device is that 
shown in Fig. 14-5a. A collar with a projecting tail, 
called a "dog," is press fit around the head of the 
bushing and is bent at the end of the tail, with one 
end resting against some part of the jig. The tail is 

r=^ -i 


a b 

Fig. 14-5. Special bushing details. 

sometimes left straight, if there is a possibility for 
the tail to strike against a lug in the same plane. 
Making such dogs involves some extra expense, but 
they are very effective in avoiding troubles with 
bushings turning and working their way out of the 
holes. The tail also provides a convenient grip for 
placing and removing the bushing. Large bushings 
may be provided with two handles for this purpose. 

One solution, which is also inexpensive, is to work 
a semicircular groove B in the edge of the head to fit 
over a pin driven into the jig plate, as shown in Fig. 
14- 5b. Although it is effective against rotation of 
the bushing it docs not prevent lifting by chips, nor 
does it facilitate the removal of the bushing. The 
arrangement shown in Fig. ]4-5b is commonly used 
for making bushings more easy to remove. A step 
A is turned down on the head, which, in this case, 
will have to be a trifle larger in diameter. This 
step permits a tool— a screwdriver, for instance— to 
be placed underneath, and with a quick jerk the 
bushing may be lifted enough to offer a good hold. 

Three methods of holding bushings to prevent 
them from turning are shown in Fig. 14-6; all of 
them on the same principle described, A shows 
a bushing with a pin inserted which then slips into 
a slot cut in the lining bushing; B shows a bushing 
with a slot milled through the collar and a pin is 
located in the jig to engage this slot; and C illustrates 
a more elaborate device that is sometimes used, 
where the stop button which is fastened to the jig 
prevents the bushing from being drawn out of the 
liner while drills or reamers are withdrawn, as well 
as preventing it from turning. 

Standardized bushing locking components have 
been developed for fixed renewable type bushings 
and for slip bushings. The fixed renewable bushing 

Fig. 14-6, Devices for preventing a bushing from rotating. 

Ch. 14 



is provided with a partly circular recess as shown in 
Fig. 14-7 and is held in position against rotation and 
push-out by a lock screw with its head engaging into 
the recess. Almost any type of screw could be used 



Courtesy ofASME 
Fig. 14-7. Standardized locking recess. 

as long as it does not have a countersunk head, how- 
ever, using a screw with a cylindrical head of sub- 
stantial dimensions, such as a socket-head cap screw 
is recommended. Preferably a shoulder screw of the 
type to be seen in Fig. 14-10, is recommended, as 
the shoulder absorbs some of the bending moment 
from the head. Head bushings without the lock 
screw recess can be locked by means of a separate 
ring-shaped clamp (Fig, 14-8) that covers the flange 
of the bushing and is provided with a recess for a 
standard socket-head locking screw. 

These locking devices require the removal of the 
lock screw before the bushing can be removed and 
are therefore not suitable for slip bushings that have 
to be changed quickly. The standardized locking 
device for slip bushings consists of a bayonet-type 
combination of a rounded notch and a curved recess, 
as shown in Fig, 14-9, and works with the lock screw 
shown in Fig. 14-10. The bushing is inserted and 
locked with a push and a twist. The notch clears 
along the head of the lock screw and the bottom of 
the recess slides under the head of the screw and 
locks the bushing. The bushing is kept in place by 
the friction from the tool as it rotates. In those rare 
cases where a left running tool is used, the recess 
must be located in the opposite direction. 

r o- 


3K ' U — W 

► H+- 

Courtesy of ASME 
Fig. 14-9. Standardized bayonet type locking recess. 



- — i 



i — 

— pi 

r— O.DI5 ft 





— H 





Courtesy ofASME 
Fig. 14-8. Standardized locking clamp. 

Lock screws are only suitable for use with flush 
mounted (headless or countersunk head type) liners, 
usually in light-duty applications. For heavy-duty 
applications clamps provide a better means of lock- 
ing the bushing against the effects of vibration and 
torque from rotation. Clamps provide a larger 
bearing surface against the jig plate and are secured 
by standard socket-head cap screws. The bending 
moment on the screw is also peatly reduced. Clamps 
can also be used for locking removable bushings in 
projected mounted liners, that is, head type lin- 
ers where the head is not countersunk. Typical 



— A « 

■t" Jl 


•» D ■ 

Courtesy ofASME 

Fig. 14-10. Standardized lock screw. 



Ch. 14 






















fea J/tSll FDR LIGHT 

/■>-,rV==n • FOR HEAVY DUTY 








Courtesy of American Drill Bushing Co. 
Fig. 14-11. Examples of clamp locks for bushings. 

examples of clamp locks are shown in Fig, 14-11; 
although they are commercially available, they are 
not standardized. 

Lock screws must be accurately located at the 
correct distance (dimension R in Fig, 14-9) from the 
liner axis. Small drill jigs for this purpose, shown in 
Fig, 14-12, are commercially available. Lock screws 

are eliminated by the use of liners with integral 

locking tabs 3 , as shown in Fig. 14-13. The con- 3 Proprietary to the American Drill Bushing Company. 

figuration of the tab is similar to a part of a standard 
lock screw, so that it engages in the locking recess of 
a standard slip bushing. In every other respect the 
liner is standard; it is pressed in place by means of an 
adapter arbor that centers in the bushing and has a 
milled slot for clearing the lock tab. 

Ch. 14 



Courtesy of American Drill Bushing Co. 
Fig. 14-12. Drill jig for lock screws. 

Nonstandard Bushings 

These are bushings of conventional configuration 
but some or all dimensions deviate from standard- 
ized dimensions. They are used where the work- 
piece presents dimensional problems for the design 
of the jig. 

The theoretical minimum center distance between 
holes is equal to the outside diameter of the bushing. 
However, the practical distance must be greater to 
allow for a metal wail between adjacent bushings 
and practical considerations require a certain mini- 
mum wall thickness to avoid uneven distortion when 
bushings are pressed in place. The center distance 
can be reduced if the bushing wall is reduced, and 
thin-wa!l bushings of the basic types are available. 
The wall thickness is approximately half the wall 
thickness of standard bushings in the normal series. 
"Extra thin" wall bushings are also available. 

When the guide bushings are very long, and, 
consequently, would cause unnecessary friction in 
their contact with the cutting tools, they may be 
recessed, as shown in Fig. 14- 14a. The distance 
H of the hole in the bushing is recessed sufficiently 
wider than the diameter of the tool so as not to bear 
on it. The length L is about twice the diameter of 
the hole, to provide guiding surfaces for the cutting 
tool which are long enough to prevent its running 
out. If the outside diameter of the bushing is very 
large compared to the diameter of the cutting tool, 
as indicated in Fig. 14-14b, the expense of making 
the bushings may be reduced by making the outside 
bushing of cold-rolled steel or cast iron and inserting 

Courtesy of American Drill Bushing Co. 
I'lg. 14-13. Slip bushing and liner with integral locking tabs. 

Kjg. 14-14, Examples of nonstandard bushings: long bush- 
ings and a bushing with an external thread. 

a hardened tool-steel bushing, mounted with a press 
fit. This bushing can be considered as a standard 
press fit bushing. The reason why a bushing may 
need to have so large an outside diameter and so 
small a hole is that it might be necessary to remove 
it for counter boring part of the already drilled small 
hole by a counterbore of large diameter, in which 
case the hole in the jig body has to be large enough 
to accommodate the large counterbore. If a slip 
bushing is longer than the lining bushing, as illustrat- 
ed in Fig. 14- 14c, it will be advantageous to make the 
projecting portion of the bushing about 1/32 inch 
(0.8 mm) smaller in outside diameter than that part of 
the loose bushing which fits the lining bushing. This 
lessens the amount of surface which must be ground, 
and, at the same time, makes it easier to insert the 
bushing, forming a point, so to speak, which will 
first enter the lining bushing. This does not interfere 
in any way with the proper qualities of the bushing 
as a guide for the cutting tool. 

In some cases, the holes in the piece to be drilled 
are so close to one another that it is impossible to 
find space in the jig for lining bushings. It is then 
necessary to make a leaf, a loose wall, or the entire 
jig, of machine steel or tool steel and harden the 
entire jig or a portion of it. 

Removable bushings are sometimes threaded on 
the outside and made to fit a tapped hole in the 
jig, as shown in Fig. 14-14d. The lower part of the 
bushing is usually turned straight, and ground in 
order to center it perfectly in the hole in the jig. 
The head of the bushing is either knurled or milled 
hexagon for a wrench. When these bushings are 
used, they are not, as a rule, used for the purpose of 



Ch. 14 

guiding the cutting too! alone, but frequently com- 
bine the functions of locating and clamping of the 
work as well. Examples are shown in Figs, 9-14, 
9- 1 5, and 9- 1 6. These bushings are not commonly 
used as slip bushings, as it would take considerable 
time to unscrew, and to re-insert into the jig body, a 
bushing of this type. 

Drills with Attached Bushings 

When machining several small holes requiring two 
or more operations, changing slip bushings becomes 
relatively time-consuming. Considerable time is 
saved by attaching them to their respective tools, so 
that they participate in the rotation and feed. Slip 
bushings used for this purpose are without heads 
and are termed "guide" bushings. Figure 14-15 
shows a guide bushing A attached to a drill. The 
free overhang of the drill must at least be equal to 
the depth of the hole to be drilled and should not 
exceed one inch in order to maintain the rigid 
support of the drill point, particularly when drilling 
a rough surface. Since the guide bushing is rotating 
against the liner, a clearance must be provided. 
To minimize the amount of frictional heat de- 
veloped, this clearance should be made as large as 
permitted by the required accuracy in the hole 
location. In drilling steel, the use of a guide bushing 
offers the advantage of providing plenty of room 
for curled chips. 

A technique that is widely used in connection 
with multiple-spindle drill heads is shown in Fig. 
14-16. A Z-shaped bracket is bored to the size of 
the drill (or other tool) and is machined on the 
outside to form a pilot which enters a drill bushing 
as the spindle is fed towards the work. 

Some drill presses have provision for the mounting 
of a bushing bracket carrying a drill bushing con- 
centric with the machine spindle. The bracket with 
the bushing is adjusted to a height slightly above the 
surface of the work. The bushing guides and sup- 
ports the drill, and the work is clamped or held in a 
positioning fixture on the drill press table. 

A combination of these techniques is found in the 
aircraft industry arid is applied to portable power 
tools used for drilling, spot facing, tapping, etc., of 
small holes in large parts. The drill jig is made of 
relatively thin metal, fiber plate, or plastic laminates 
and neither the drill jig, the bushings, nor the drills 
are capable of guiding and stabilizing the rather 
heavy portable power tool. The operation of this 
type of equipment is shown in Fig. 14- 1 7a. The 
bushings in the drill jig are liners. They are secured 
with a nut on the far side of the jig and are provided, 

f — <^P 

i r 




,_oi.Ay r - ! h auSHii 

BORE IN J 10, 


Fig. 14-15. (Left) A drill with a guide bushing attached. 

Fig. 14-16. (Right) Stationary guide for multiple drilling 

and reaming tools. 

directly or indirectly, with two locking prongs on 
the forward side. The drill bushing is a long and 
heavy bushing (the tip) that is screwed into the nose 
of the power tool; it carries a flange with two pro- 
jecting locking lugs matching the prongs on the jig 
bushings. In essence the tip is a slip bushing 
mounted on a power tool; when in use the tip is 
pushed all the way into the liner and the power 
tool is rotated counterclockwise so that the lugs 
engage the prongs. The power tool is now positioned 
and is so well supported that it takes little effort on 
the part of the operator to hold it up and operate it. 
The drill is rotated and fed through the tip onto the 

The locking prongs can be integral with the jig 
bushing and for light-duty work they can be made 
of two ordinary lock screws of the type shown in 
Fig. 14-10. Individual lock buttons can be used 
instead of lock liner bushings where the holes are 
spaced too closely. When holes are closely spaced 

Ch. 14 





Courtesy of American Drill Bushing Co. 
Fig. 14-17. (a) Operation of portable power drill with thin-walled drill jig. Liner bushings are equipped with individual 
locking prongs for drill spindle. <b) Pair of locking strips for drilling of holes, closely spaced in line. 

in a line, locking is done by two undercut locking 
strips along the line of holes as shown in Fig. 14-1 7b. 

Empirical Formulas for Design of Bushings 

Very wide, very long, and very large bushings are 
the three most common types of nonstandard bush- 
ings that must be individually designed and specially 
made. Very wide bushings are for sequences of 
operations such as those shown in Fig. 14-18, where 
one hole is drilled to two diameters, or where a 
drilling operation is followed by counterboring, 
countersinking, or spot facing to a diameter signifi- 
cantly larger than the hole diameter; perhaps fol- 
lowed by reaming. The diameter of the liner is 
slightly larger than the diameter of the counterbor- 
ing tool. The drill bushing fits the liner with the 
sliding fit for slip bushings. The hole diameter in the 
bushing is the drill diameter with normal clearance, 
and other dimensions on this bushing can be cal- 
culated by the formulas below. The counterbore 

requires no bushing since it is guided by the pilot 
in the drilled hole. The reamer bushing differs from 
the drill bushing only in hole diameter. 

Very long bushings are of one of the types shown 
in Fig. 14-4b and Fig. 14-14. The letter symbols 
used in the following formulas are the standard 
letter symbols from Fig. 14-2 plus the following: 
Bearing length = L, Wall thickness = T = 1/2 (B-A) 
and Flange width on head = G= 1/2 (E-B). 

The basic dimension is the hole diameter A. 
According to an old rule-of- thumb, the bearing 
length can be taken as 

L =s 2A 

This bearing length will generally work well. It is 
long enough to provide sufficient bearing surface but 
not long enough to cause excessive friction. How- 
ever, it is larger than is needed for large drills, while 
small drills, from 1/4-inch (6-mm) diameter on 
down, can well use a longer support. A more 
sophisticated approach would be to take: 

Inch Dimensions Millimeter Dimensions 

L = \Ta + 0.4 


L = 5y/~A + 10 


for tools with sharp edges, such as drills; and 

L = 0.8 n/T+ 0.4 inch L = 4>/~A+ 10 mm 

for tools with smooth shanks, such as boring bars, 
rose reamers, etc. 

For minimum wall thickness, take: 

T= 0.2 \/~A + 0.04 inch r = v7+lram 

Courtesy of Technological Institute, Copenhagen 
Fig. 14-18. Examples of very wide bushings. 

Body diameter will then be: 

B=A + IT, or 
B = A + 2T + 1/32 

B = A + 2T, or 
B =A + 2T+0.8 mm 



Ch. 14 

depending on whether or not the bushing is counter- 
bored (Fig. 14- 14a.) 

For the corner radius at the inlet end, take: 
B » 0,1125 VT inches D = VT mm 

For the normal height of the head, take: 

F ;= 0.6 VT inches F=7vTmm 

This value docs not apply to bushings of the type 
shown in Fig. 14-4b. 

Inch Dimensions 

For the diameter of the head, take: 

E = B+F- 1/8 inch forU< 1/2 inch, and 

£= B + F- 1/16 inch for£> 1/2 inch 

where F is calculated by the previous formula for 
inch dimensions. 

For the length of seat, take: 

B<C<3B for B< 3/8 inch 

0.67B<C<3B for 3/8 <B<^ 3/4 inch 

0.6B<C<2B for B> 3/4 inch 

The range of values for C corresponds approxi- 
mately to the values in the ANSI Standard. Calcu- 
lated diameters are converted to multiples of 1/64 
inch and calculated lengths are converted to multi- 
ples of 1/16 inch. Other calculated dimensions are 
converted to the same selection of fractions as are 
used in the ANSI Standard. 

The following fits based on ANSI Standard 
Tolerance limits (from ANSI B4. 1-1967), are 

for liner in jig plate (press fit) H7-n6 

for slip renewable bushing in liner F7-m6 
for fixed renewable bushing in liner F7-h6 

Tolerance limits for H7, n6, and h6 are found in 
reference books, such as Machinery's Handbook f 
tolerance limits for F7 and m6 are found in ANSI 
B4. 1-1 967, Appendix 1, p. 16 and p. 19. 

ixamp/e- Calculate dimensions of 3/4-inch diame- 
ter drill bushings of the types shown in Figs. 14-4b 
and 1 4- 14a, 

A = 0.750 inch; vT = 0.8660; 

VT = 0.9086; VT = 0.9306 

Bearing Length : 
For Fig. 14-] 4a 

i=Vo.750 +0.4 = 1.2660^1 5/ 1 6 inches 
Corner Radius: 

Eric Oberg and l-.D. Jones, Machinery ',? Handbook (New 
York: industrial Press Inc., 1971) 19th ed„ pp. 1523 
to 1525. 

D = 0. 11 25 V 0.750 = 0. 1 022 ^ 3/32 inch 

Total Length : 
For Fig. 14-4b 

L + D= 1.2660+0.1022= 1.3682*= I 3/8 inches 

Wail Thickness: 

T = 0.2Vo.750 + 0.04 ■ 0.2132 inch 

Body Diameter: 
For Fig. 14-4b 

5 = 0.750+ 2X 0.2132= 1.1764 *= 1 12/64 
1 12/64= 1 3/16 inches 

For Fig. 14- 14a 

B =0.750 + 2X 0.2132+ 1/32= 1.2077 
1.2077 * I 7/32 inches 

Height of Head: 
For Fig. 14- 14a 

F = 0.6 Vo.750 = 0.5584 « 9/1 6 inch 

Diameter of Head; 

E m 1,2077 + 0.5584 - 1/16 = 1.7036 
1.7036 *» 1 45/64 inches 

Length of Seat: 

0.6 X 1.2077 <C<; 2 X 1.2077 

0.7246 <C< 2.4154 

3/4 inch < C < 2 3/8 inches 

Ch. 14 



Millimeter Dimensions 
For the diameter of the head, take: 

E = B + F — 3 mm for B < 1 3 mm and 

E = B + F- 1.5 mm forfl>13mm 

where F is calculated by the previous formula for 
millimeter dimensions. 

For the length of seat, take: 

B<C<ZB forB< 10 mm 

Q.67B<C<3B for 10 mm <S< 19 mm 

0.6B < C -g IB for B > 1 9 mm 

For millimeter dimensions, the following fits based 
on ISO Recommendation R286, are recommended: 

for liner in jig plate (press fit) H7-n6 

for slip renewable bushing in liner F7-m6 
for fixed renewable bushing in liner F7-h6 

The tolerance limits are found in reference books, 
such as Machinery's Handbook. 

rJjramp/e-Calculate the dimensions of 19-mm-dia- 
meter drill bushings of the types shown in Figs. 
14-4band 14- 14a. 

A= 19 mm;V~A =4.36;V r A = 2.67; \f\ = 2.09 

Bearing Length: 
I = 5\AT9 + 10 = 31.8^32 mm 

Corner Radius: 

3 I 

D = V 19 = 2.67^3 mm 
Total Length : 

L+D = 32 + 3 = 35 mm 
Wall Thickness: 

T = s/~[9 + 1 =4.36+ 1 = 5.36 *= 5.5 mm 
Body Diameter: 
For Fig, 14-4b 
B= 19+ 11 =30 mm 

For Fig. 14- 14a 
5= 19+ 11 + 0.8 = 30.8 =« 31 mm 

Height of Head: 

For Fig, 14-14a 


F=7V 19 = 14.63 =s 15 mm 
Diameter of Head: 

£■ = 31 + 15- 1.5 = 44.5 mm 
Length of Seat: 

0.6 X 3KC<62 

*lbid.,pp. 1532 to 1537. 

19mm<C< 62 mm 

Materials for Bushings 

Bushings are generally made of a good grade of 
tool steel to insure hardening at a fairly low tem- 
perature and to lessen the danger of cracking during 
heat treatment. Typical examples are the oil-harden- 
ing cold work tool steels, types 01 and 02 and AISI 
521 00, heat-treated to a hardness of 63 ± 2 Rockwell 
C. They are also made of carbon steel of at least 
0.6 percent carbon content. They can also be made 
of machine steel with a lower carbon content, which 
will answer all practical purposes, provided the 
bushings are properly case-hardened to a depth of 
about 1/16 inch (1.5mm). Typical examples are 
AISI 1144 and AISI 8620, The hardness required 
is 790 ± 50 Vickers, which is approximately equiv- 
alent to the Rockwell hardness quoted above. Very 
large bushings are made from steels in the 4100 
series. Bushings are also available in a higher quality 
level, made of a high chromium, high carbon die 
steel. These bushings will outlast ordinary bustlings 
5 to 6 times. Finally, bushings are also made of 
sintered tungsten carbide, Class C-2, the straight 
cobalt grade with 6 percent cobalt and 94 percent 
tungsten carbide, with a hardness of 92 Rockwell A, 
The total length of the bushing body is made of 
carbide, while the head is made of steel and is 
copper brazed around the upper part of the bushing. 
The life of these bushings is about 50 times longer 
than the life of ordinary bushings. Bushings are 
made to the same quality level from a sintered 
ferrous titanium carbide composition that is machin- 
able and heat-treatable to a hardness of 71 Rockwell 
C. Bushings for guiding tools sometimes may be 



Ch. 14 

made of cast iron, but only when the cutting tool is 
so designed that no cutting edges come within the 
bushing itself. For example, bushings used simply 
to support the smooth surface of a boring-bar or the 
shank of a reamer might, in some instances, be made 
of cast iron. But hardened steel bushings should 
always be used for guiding drills, reamers, taps, etc., 
when the cutting edges come in direct contact with 
the guiding surfaces. If the outside diameter of the 
bushing is very large, as compared with the diameter 
of the cutting tool, the cost of the bushing can 
sometimes be reduced by using an outer cast-iron 
body and inserting a hardened tool-steel bushing, as 
seen in Fig. 14-1 4b. 

Special Bushings 

Many jigs are now made of materials other than 
steel and cast iron. The commonly used materials 
in this category are cast or laminated plastics, plastic 
or fiber sheets, aluminum and magnesium sheets, and 
tooling plate. These materials do not permit con- 
ventional press fit mounting of bushings. 

In jigs made of cast or laminated plastics, bushings 
are either cast in place as the jig is made, or potted 
in cavities that are formed by cores as the jig is 
laminated. In either case, the bushings are made 
with an outer surface texture, or pattern, that per- 
mits the plastic to grip and lock the bushing. In 
each case the surface configuration is such that the 
bushing is positively locked against rotation as well 
as against axial displacement. These patterns are a 
combination of deep circular grooves with shallow 
and sharp longitudinal serrations or polygonalflanges, 
or are systems of two sets of V-grooves crossing each 
other to form a pattern of diamond-shaped pro- 
jections similar to a knurled surface with a coarse 
pitch. Samples of these patterns are shown in 
Fig, 14-19. 

These bushings are also available with a 0.0 30- inch 
(0.8-mm)- thick ceramic coating to act as a heat 
barrier for the protection of the potting or bonding 
material against the frietional heat developed within 
the bushing. Other jig materials, such as aluminum, 
magnesium, Masonite®, and even wood, which is oc- 
casionally used for lightweight jigs, can be equipped 
with bushings that are specially designed for press 
installation. The lower half of the bushing is cylin- 
drical and precision ground for locating the bushing 
in the mounting hole. The upper half is larger in 
diameter and has longitudinal serrations (see Fig. 
14-19). The step in diameter prevents axial dis- 
placement, and the serrations cut into the jig ma- 
terial and lock against rotation. Template bushings 

:: piiiiu 

Courtesy of American Drill Bushing Co. 
Fig. 14-19. Bushings for use in drill jigs made of nonfenous 
and nonmetallic materials. 

are those used for template tooling, that is, jigs 
made from metal plates in thicknesses from about 
1/16 to 3/8 inch (1.5 to 10 mm). This jig material is 
too thin for conventional press fit and for serrated 
bushings. Template bushings also require a fastener, 
which can be a nut or a deformable fastener. An 
example is shown in Fig. 14-20, The hole in the 
template (the jig plate) is drilled, reamed to 0.001 to 








Courtesy of American Drill Bushing Co. 
14-20, Template bushings. 

Ch. 14 



0.003 inch (0.03 to 0.08 mm) oversize, and counter- 
sunk 90 degrees. The bushing is placed in the hole, 
and an aluminum locking ring is crimped around it 
and into the groove. The crimping is done with a 
special adapter arbor in an arbor press, or with a 
rivet gun. Bushings of this type can be removed by 
cutting the locking ring, and then can be reused. 

Circuit board bushings are small bushings for drills 
of 1 /4-inch (6-mm)-diameter, down to #80 (0.35mm) 
for the drilling of circuit boards. With standardized 
hole diameters, they are available in a variety of 
outside configurations to match the circuit board 
drilling machines currently on the market (see 
Fig. 14-21). 

Courtesy of American Drill Bushing Co. 
Fig. 14-21. Circuit board bushings. 



Design of Fixture Bodies 


At this stage the choice of locating and clamping 
devices and intermediate supports, if these are 
required, has been finalized. A drawing is made 
showing these devices in their correct position rela- 
tive to the part. The part outline is also shown. 
Using a color code for the lines, to differentiate 
the various items, is helpful. 

Tooting holes are those drilled in the part for the 
purpose of locating it in a fixture or in a series of 
fixtures, one after another. All other dimensions 
are directly or indirectly referenced to the tooling 
holes. Where tooling holes are used, they must be 
shown and identified. 

The drawing, as it now stands, is a phantom 
drawing with the details floating unsupported; the 
next step is to outline the fixture body so that it 
connects all loose parts and satisfies several other 
requirements as well. 

The Use of Existing Components 

Regardless of the type of fixture, the first design 
step is to examine the possibility of using existing 
equipment or components. The most promising of 
these is to use a manually or air-operated machine 
vise, with jaw inserts. The vise can be used as a 
base for milling and planing fixtures and for drill 
jigs, in which case it is also provided with a jig plate. 
In the case of a drill jig, the next possibility is the 
universal drill jig, or "pump" jig. It supplies the jig 
structure and clamping mechanism and needs only 
to be provided with locating devices and a jig plate 
with bushings. The use of this type of jig is described 
in detail in Chapter 21 . The third possibility, appli- 
cable to box-type drill jigs, is the use of a commer- 
cially available jig box, an example of which, 
equipped here as a leaf jig, is shown in Fig. 15-1. 
These boxes are made of cast iron or of aluminum 

Courtesy of Vlier b'xgineeritig Corp. 
Fig, 15-1. Commercially available box-type drill jig with leaf. 

with cast-iron corner posts, and are available in sizes 
up to 4 by 8 inches (100 by 200 mm), and 6 by 6 
inches (150 by 150 mm). Plain fixture bases are 
made in the two styles shown in Fig. 15-2. The 
difference lies in the location of the lugs. They are 
applicable to fixtures and jigs of many types, and are 
available in sizes up to 12 by 1 8 inches (300 by 450 
mm). In any case, the final make-or-buy decision 
is based on a cost estimate. If commercial compo- 
nents do not fit, however, the design procedure 

Drill Jigs 

In the case of a drill jig, the designer has three 
choices: A plate jig, an open jig, or a closed jig. 
If all holes are parallel and drilled from one side, 
and the part is large and stable, the plate jig is the 
probable solution. If all holes are drilled from one 
flat surface, the plate jig takes the simplest possible 
form, a flat plate. If the holes are located in surfaces 
at different levels, the designer has the choice of 
making a flat plate jig with projecting bosses of 
varying lengths, or to form the plate with bends and 
offsets so that it follows the contour of the part 


Ch. 1 5 



Courtesy of Standard Parts Co. 
Fig. 15-2. Two styles of commercially available fixture bases, predominantly used for milling fixtures. 

surface. The first possibility is usually recommended, 
except in extreme cases. 

[f the part is small and not easily supported, and 
all holes are parallel and drilled from one side, an 
open jig is the solution. If there are holes in more 
than one direction, a closed box jig is needed. The 
design procedure for these two cases is described 
in detail in Chapter 18, Drill Jigs. All drill jigs, 
with the exception of plate jigs, must have feet 
which can be either attached to the jig body, or 
integral with the jig body. The preferred forms of 
integral feet are the L and T; their overall dimen- 
sions must be large enough to bridge the width of 
the slots in the machine tool table. 

Fixture Clamping 

All other types of fixtures require a fixture base 
that is aligned with and clamped to the machine 
tool table. The fixture base must always have slots, 
not holes, for the clamping bolts so that the nuts do 
not have to be completely unscrewed to remove 
the fixture. The traditional form for the clamping 
bolts is the T-bolt or screws with T-nuts, which 
must slide all the way to the end of the table to be 
removed from the T-slots. There are commercially 
available clamping nuts and bolts, however, that can 
be lowered into the T-slot and rotated into the 
gripping position. 


Alignment is obtained in principle by means of a 
key in the fixture and an alignment slot in the ma- 

chine table. While T-bolts and nuts must have a loose 
sliding fit in the T-slot, a key must have a close fit in 
its slot, and the fixture designer must have the data 
for the dimensions of alignment slots in the machine 
tables for which he is designing the fixtures. Most 
machine tables do not have separate alignment slots, 
but the T-slots serve both purposes. Since the 
clearance in a T-slot may vary, it is a firm rule that 
a fixture must align against only one side of the T- 
slot. Several types of combination clamping and 
aligning devices are commercially available. A 
representative example of an aligning clamp that is 
not proprietary is shown in Fig. 1 5-3, 

The construction of the clamping device is as 
follows; Fitting into the conventional T-slot in the 

Fig. 15-3. A nonproprietary, self-aligning fixture clamp. 



Ch. 15 

machine table is a hardened and tempered, cast-steel 
locating T-block of rectangular cross section with one 
edge of its tongue machined to provide a flat 
positioning ledge inclined at an angle of 60 degrees 
from the horizontal plane. The block should be a 
snug sliding fit within the T-slot, with clearance 
kept as small as possible. A standard, square- 
headed clamping bolt is inserted through a hole in 
the center of the T-block, with the head of the bolt 
fitting closely into a slot machined across the under- 
side of the T-block. 

A cylindrical cam-sleeve with a sliding fit in a hole 
bored through the lug of the fixture is drilled to pro- 
vide a clearance of at least 1/32 inch (0.8 mm) for the 
clamping bolt. Two flats are machined on the lower 
end of. this sleeve, with the flat on the left machined 
so that it will be in vertical alignment with the left- 
hand edge of the T-block. The lower portion of the 
flat on the opposite side of the sleeve is inclined 
at an angle of 60 degrees, as shown, to mate with 
the positioning ledge on the T-block. Both the left- 
hand flat and the right-hand inclined surfaces should 
be hardened and polished, since they are the parts 
most subject to friction and wear. A clearance of 
1/16 inch (3 mm) should be provided between the 
lower face of the sleeve and the top surface of the 
T-block, as indicated. 

The upper end of the cam-sleeve, which projects 
beyond the top face of the fixture lug, is provided 
with a fine-pitch external thread to accommodate 
the circular ring-nut. The periphery of the ring-nut 
is knurled to facilitate manual rotation. A standard 
hexagonal lock-nut is screwed on the upper, threaded 
end of the clamping bolt. 

In the illustration, the fixture and parts of the 
clamp are shown in the correct relative positions 
they will take when the fixture has been properly 
located and clamped to the machine table. The 
cam sleeve has been moved downward by simply 
tightening the lock-nut. This movement causes the 
inclined flat on the right-hand side of the sleeve to 
contact the positioning ledge on the T-biock, thus 
pressing the fixture toward the left until it is stopped 
by the flat on the left-hand edge of the sleeve coming 
into contact with the side of the T-slot in the machine 
table. Before tightening the lock-nut, the ring-nut 
should be backed off slightly to clear the top sur- 
face of the fixture lug and allow the sleeve to pass 
through the hole in the lug. 

When the fixture is aligned , the ring-nut is tightened 
-by hand pressure only-and the lock-nut is then 
given a final, partial turn to insure rigid clamping and 
positive alignment of the fixture with relation to 
one side of the T-slot. The cam-sleeve is prevented 

by the ring-nut from pressing too forcibly against 
the positioning ledge on the T-block. 

The Fixture Body 

The body of the fixture is now built up from the 
base. It consists, essentially, of walls, forming a chan- 
nel or a complete box (a pot, in the case of a lathe 
fixture), or of individual uprights or brackets. Again, 
the phantom drawing shows where material is 

The principal consideration, apart from rigidity and 
strength, in the application of material is clearance. 
At this stage, clearance is easily arranged; later, it 
may be unavailable. Clearance is required at the 
following places: Around the part to allow for 
dimensional tolerances, around the path of the part 
as it is being loaded and unloaded, and around the 
fixture to prevent collision with any part of the 
machine. Wherever the operator's hand is applied, 
a finger clearance of at least 5/8 inch (16mm) must 
be provided; more, if a full hand-grip on the part is 
anticipated. No part of the fixture should obscure 
the view of the cutter's action area. If possible, 
the locating areas should be visible. Windows in 
side walls may be needed that also serve to reduce 
the weight and to provide access for chip cleaning 
and for the free flow of cutting fluid. Inaccessible 
pockets that can accumulate chips must be avoided. 
Projecting points, corners, and edges are a hazard to 
the operator; these musl he blunted and rounded. 

Fixtures that require little handling are made of 
steel or cast iron; those that require a great deal of 
handling are made of a selection of lightweight 
materials now available, such as: aluminum, magnes- 
ium, cast or laminated plastics, and plastic or fiber 

Three Construction Principles 

A fixture body may be of the built-up type, a 
casting, or a weldment. Historically, the first two 
types have been used from the inception of the use 
of fixtures, with cast fixtures dominating; the arrival 
of the welding process, particularly arc welding, 
has changed the picture, and today the welded 
fixture is the dominant type. It has, however, 
not completely eliminated the two older types for 
each has its advantages and therefore its limited 
area of application, therefore a discussion of the 
principles and merits of all three types is justified. 

Typical Examples 

As an introduction, it will be interesting to see 
how the three construction principles can be ap- 

Ch. 15 



plied to the same assignment. Already the simple 
case of a channel fixture, as shown in Fig. 15-4, 
demonstrates some principal features. It is obvious 
that the rigidity of the three designs increases in 
the sequence: built-up, welded, cast; because the 
cast channel is fully integral, the welded channel 
is partially integral, while the built-up channel de- 
pends for its rigidity on the size and number of 
fasteners. A screw joint is never completely solid 
because of the required hole clearances, and any 
screw joint in a built-up fixture must therefore be 
additionally and permanently secured by means of 
tightly fitting dowel pins, as shown. The three types 
differ also with respect to "clean contours." The 
built-up fixture, if made from fully machined or 
cold-finished components, presents well defined 
inner and outer contours with clean inner corners. 
The welded fixture has projecting weld beads in the 
inside corners and on the outer sides. The cast 
fixture has rounded fillets in the inner corners and 
clean, but not parallel, outer sides because of the 
draft. These features require consideration in the 
planning and layout of areas to be machined. 


t\ if. 



l ; ig. 1 5-4. Three designs of a channel fixture, 

A somewhat more complicated case is the box jig 
with hinged leaf shown in Fig. 15-5, a through c. It 
is assumed that the three jigs must have a machined 
base surface of dimensions A X B, as shown. The 
three jigs are drawn to the same scale and the thick- 
nesses shown are representative. The built-up jig, 
Fig. 15-5a, is made of mild steel plate. It is de- 
signed strictly to dimensions A and B, because the 
components are machined before assembly. All 
fixed joints are assembled and secured with screws 
(shown as larger circles) and dowel pins (shown as 
smaller circles). The screws provide the forces that 
hold the pieces together, but since screw holes 
normally are drilled with a clearance around the 
screws, they do not guarantee the exact position of 
the parts relative to each other. The dowei pins 
secure the parts in their exact position. Therefore, 

dowel pins are made to fit exactly in their holes. 
Normally, it takes two pins to secure a part, and for 
best control of the position, the pins are located as 












b - 








Ch. 15 

far apart as possible; therefore, dowel pins are 
usually placed diagonally opposite each other. In 
the present case, the base is closely fitted into 
grooves in the end walls; therefore, one pin would 
be sufficient, theoretically, at each end of the 
fixture; however, most designers would choose two 
pins in accordance with traditional practice. Where a 
removable part is to be secured with dowel pins, the 
pins are mounted with a press fit in the fixed part, 
and holes in the removable part are made with a 
sliding fit over the pins. Thicknesses of the 
individual parts are selected approximately equal to 
those in the cast fixture to provide sufficient 
bearing surfaces in the joints. The two straps 
improve the rigidity against longitudinal forces 
while they retain accessibility to the base for chip 

Dowel Pin Applications 

Dowel pins, are used extensively in all categories 
of tooling, and the correct design and application 
of these small components is of fundamental im- 
portance. Dowel pins are made of soft steel or 
drill rod. A hardened dowel pin can be made from 
commercial hardened drill rod. Standard dowel pins 
can be purchased either soft or hardened, and are 
made with a 5 degree taper at the leading end for 
easy and safe start. For hardened dowel pins, the 
surface hardness is 60 to 64 Rockwell C, the 
core hardness 50 to 54 Rockwell C. The shear 
strength ranges from 150,000 to 210,000 pounds 
per square inch (1035 to 1450 N per square milli- 
meter). Diameter tolerance is plus and minus 0.0001 
inch (0.003 mm) with a surface roughness of 4 to 6 
microinches (0.10 to 0.l5jum). Normally, they are 
manufactured with 0.0002-inch (0.0005mm) over- 
size to provide a secure press fit, but are also avail- 
able with 0.001 -inch (0.005 mm) oversize for repair 
work in cases where a hole has been worn or acci- 
dentally machined oversize. 

As a general rule for jigs and fixtures, the dowel 
diameter is selected one size smaller than the 
assembly screws. For presswork dies, dowels are 
made the same size as the screws because of the 
conditions of shock and vibration under which the 
dies operate. The length of engagement, or the 
bearing length, i.e., the length which the pin 
protrudes into the second member of the assembly, 
is m to 2 times the dowel diameter. Soft dowel 
pins can be used for plain locating purposes where 
no heavy load is applied to the pin. However, 
hardened pins are sometimes preferred because they 
are usually ground to closer tolerances. A locating 

dowel pin that is also subjected to a severe shear 
load should always be hardened. 

Locating the Dowel Pin 

Dowel-pin locations are not usually specified by 
dimensions, but are shown by center lines on the 
drawing. The toolmaker usually knows that he is to 
locate the holes somewhat at random in the area of 
the center lines; a note to this effect is sometimes 
placed on the drawing. An exception occurs when 
the dowel holes are to be jig ground, an operation 
which is not commonly practiced on jigs and 
fixtures, but is occasionally, on dies. Dowel pins 
should be so located that the assembly of the 
components is foolproof. Symmetrical parts can be 
located in more than one relative position, and 
dowel pins are used to make certain that they go 
together in the one and only correct location. 

In an assembly where one component must be 
removed frequently, the use of dowel pins together 
with straight drill bushings is sometimes recom- 
mended. Using a hardened pin in a hardened 
bushing, results in a precise and wear-resistant fit. 

The general rule is that dowel pins should be so 
located that the holes pass entirely through the two 
components. This is done for easy removal of the 
dowel pin when needed. If, however, a dowel pin 
necessarily must be pressed into a blind hole, then 
the hole should be drilled deeper than is required to 
hold the pin. A good practice (which is not always 
followed) is first to drill the hole deeper and then to 
ream it to the depth to which the pin must 
penetrate. It is not possible to exactly specify just 
how much deeper the dowel-pin hole should be 
drilled because this depends upon the restrictions 
inherent in the design of the component into which 
it is to be drilled. The purpose of drilling the blind 
hole to a greater depth is to reduce the build-up of 
air pressure behind the pin, in the bottom of the 
hole. The pressure build-up follows Boyle's Law, 
but does not have to be calculated. 

Assembly Screws 

Assembly screws are usually hexagonal socket 
head cap screws made of a high-strength steel. 
The minimum length of engagement of the screw 
thread should be as follows (where D is the screw 


in steel 
in cast iron 
in magnesium 
in aluminum 

in fiber and plastic 

1 Vi X D 

2 XD 
2Vi X D 

3 X D and up 

Ch. 1 5 



The welded jig in Fig. lS-5b, is also made of mild 
steel plate. The inherent rigidity in the welded joints 
permits the use of thinner plates. On the other 
hand, the inner space must exceed the A and B 
dimensions with sufficient clearance to avoid 
removing the weld beads in machining. 

The cast jig in Fig. 15-5c is designed with larger 
material thicknesses than the welded jig, since cast 
iron has less tensile strength and a lower modulus of 
elasticity. The part is fully monolithic and, 
therefore, has no weak areas. Again, the machining 
of the base requires full clearance all the way 
around. The cast design, however, requires less 
machining than the other two designs, and needs no 
cutting and fitting. In accordance with most 
common drawing practice, the jig is drawn without 
showing the draft, but when proper draft is 
provided on all vertical surfaces, the casting can be 
made from a single pattern. In this case, the side 
wails would be solid, which is excellent from a 
structural viewpoint, but sacrifices access to the 
base for chip cleaning. Should this point be 
essential, either machined or cast windows could be 
provided, as indicated by the chain-dotted lines. 
Forming windows in the casting is perfectly 
possible, but requires the use of cores, thus 
complicating the foundry work. No evaluation or 
choice between the three pinciples will be made 
here on the basis of these two simple examples, 
because such a choice would depend on many 
factors, such as size, available equipment, time, etc. 

Built-Up Fixtures 

For each of the three design types, there exists 
some general rules and recommended practices 
which may provide useful guidance in the design. 
Built-up fixtures offer the greatest freedom in the 
design, essentially because there are no thermal- 
metallurgical limitations involved. The material is 
usually low-to-medium carbon steel, from AISI 
1025 to AISI 1040; steels with very low carbon 
content do not machine well to produce smooth 
surfaces. Hardened or otherwise heat-treated steel 
can, without difficulty, be assembled with softer 
steels when desired. Small fixture bodies may be 
made in one piece by machining (carving) them 
from a block of steel. The whole body may be 
heat treated, thereby eliminating the need for 
separate hardened components such as drill bush- 
ings, locating points, etc. 

About the only two rules regarding material 
thicknesses refer to the stability and strength of 
joints. Experience has shown that when thicknesses 

are selected as if they were intended for castings 
(see later), they will usually provide sufficient 
bearing areas for stiffness, and they preferably 
should be two times the OD of the assembly screws 
used, with 1.6 times as the absolute lower limit. 
There are no upper limits. 

When additional rigidity is needed, it is necessary 
to use straps, as shown. Gusset plates are not prac- 
tical in built-up fixtures. A number of different joint 
patterns are shown in the composite structure in 
Fig. 15-6. A different example is shown in Fig. 15-7, 
consisting of a channel bracket mounted on a plate 
flange. The channel is machined from a solid block. 
Standard commercial rolled sections offer but little 
opportunity for use in built-up fixture construction 
because of their rounded fillets and thin wall 
thicknesses. Therefore, where channels and angles 
are needed in a built-up jig, they will have to be 
machined from the solid block. As a rule-of-thumb, 
this method can be assumed to be economical for 
dimensions up to 2 to 4 inches by 8 to 12 inches 
(50 to 100 mm by 200 to 300 mm). Beyond these 
dimensions, it is cheaper to weld them. 

Contoured flat components can be cut advanta- 
geously from plate stock on a contour handsaw 

Fig. 15-6. A composite fixture structure showing different 
joint patterns. 



Ch. 15 

Fig. 15-7. A channel bracket mounted on a plate flange. 

Fig. 15-8. A bracket type drill jig made of flame-cut plate. 

or with a cutting torch, and machined afterwards 
where necessary. An example is seen in Fig, 1 5-8, 
showing a drill jig of the bracket type. The built-up 
principle offers the advantage of having the top 





Fig. 15-9. A jjg made of one flame-cut plate and two straight 

side of the base machined before assembly, an 
operation that would be more difficult if the jig was 
welded together. Modified approaches to related 
problems are shown in Figs. 1 5-9 and 15-10. 



TtT l ■ » 

mi i I m 

i n 
( — rr 

I L I 

Fig. 15-10. A modification of the jig shown in Fig. 15-9, 
made entirely of straight plates. 

Cast Fixtures 

The foundry trade has a large bag of tricks and 
devices by which it can solve almost any design 
problem, and the use of castings, therefore, presents 
a great flexibility of form to the designer. However, 
these devices have their price; there are a few rules 
to which a casting must conform in the interest of 
economical production. These concern the easy 
withdrawal of the pattern from the mold, the free 
flow of metal in the form, uniform shrinkage, and 
the avoidance of "hot spots." 

The first condition can be stated as "no undercuts 
with respect to the direction of withdrawal." Every 
fixture has, in a sense, a work space in the form of a 
more or less enclosed cavity for receiving the part, 
also, a form that permits easy and unobstructed 
loading and unloading will, usually, also permit easy 
withdrawal from the mold. Exceptions occur when 
the fixture has localized projections or depressions 
perpendicular to the direction of motion. One 
example was the window indicated in the cast 



Fig. 15-11. A bracket fixture in cast design. The design to 
the left requires a split pattern; the design to the 
right is made from a one-piece pattern. 

Ch. 15 



fixture in Fig. 1 5-5c. Another example is the 
bracket fixture seen in Fig. 15-11. The design to the 
left, with a circular boss and a machining clearance 
groove in the base, requires a split pattern and the 
mold to be parted along a-a. By the small changes 
shown in the design to the right, the parting line can 
be b-b, and the pattern can be made in one piece. 
Numerous examples of this and similar concepts are 
found in foundry literature. Only a few examples 
with direct reference to fixture design shall be given 

A box-type fixture with a dividing wall may be 
designed as in Fig. 15-1 2a. This requires two cores, 
which are eliminated by either one of the designs in 
Fig. 15-1 2b and c. If the purpose of the upper 
flange is additional strength, this is compensated for 
in design b, by increasing the thickness in the 
dividing wall. If the purpose is to provide a flat 
surface for assembly with other components, then 
this is accomplished by design c. 


Fig. 15-12. Three different designs of a box type fixture. 
View a requires two cores; views b and c can be 
made without cores. 

A Telated case is seen in Fig. 15-13. The box to 
the left requires a core, while the box to the right 
can be formed without a core and is, for all 

n a 

Fig. 15-13. Two different designs of a flanged channel jig. 
The design to the left requires a core; the 
design to the right is made without a core. 

purposes, at least equivalent to the design at the 
left; perhaps even better, as it eliminates two metal 
accumulations in the two T's, 

A prototype for a widely used class of drill jigs- is 
the casting shown in Fig. 15-14. It contains a 
number of details, providing for angular feet on the 
top and bottom surface, and long straight strips that 
can serve as feet on all four sides. Nevertheless, with 
the necessary draft, the pattern can be withdrawn 
and the casting made without cores. If it is now 
desired to reduce the length of the strip feet by 
cutting back as indicated at A, and to make these 
feet angular by adding horizontal ribs fi.then the 
feature of pattern withdrawal in this casting is lost, 
and four cores of two different shapes will be 


c a- 



Fig. 15-14, A typical drill-jig casting. 

Rules for Dimensioning Cast Fixtures 

The condition "free metal flow" is essentially 
equivalent to the setting of a lower limit to the 
metal thickness in walls. If below such a limit, the 
metal will suffer excessive heat loss and solidify 
before the wall cavity is properly filled, forming 
what is known as a "cold run." These lower limits 
depend on the length of flow for the metal and, 
therefore, on the size of the part. Broadly, the 
following values are quoted: 



Ch. 15 

For average size 

castings 3/8 to 1 /2 inch ( 1 to 1 3 mm) 

For smaller 

castings 1/4 to 3/8 inch (6 to 1 mm) 

For very small 

castings down to 1 /8 inch (3 mm) 

The wall thickness can be more specifically related 
to the overall dimensions. In many cases, a wall 
serves as the web in a beam, either an I-beam, a 
T-beam, or an angle, as indicated in Fig. 1 5-15. With 
beam height H, the thickness / can be taken as 

t — 0.2 \/H inches 

= v f /7i 

and for double web beams, as indicated in Fig. 
15-16, the thickness in each web can be taken as 

t = 0A6y/H inches 


0.8 y/H 


The reason for this thickness reduction is twofold; 
with double walls, the temperature in the mold is 
higher and meta! can flow satisfactorily in a thinner 
wall cavity without cold run; statically, the double 
web beam has sufficient strength with less web 



- ■ — 1 



Fig. 15-15. Dimensions of open beams. 

In many cases, a flat plate within a casting forms a 
series of panels within a frame. Examples are 
indicated in Fig. I5-17.The length L between cross 
members (ribs, dividing walls, etc.) may be taken 
into consideration by taking 

f=l/4+ 1/15 y/T inches 


t = 6+-jy/L 


The calculated thickness should be interpreted in 
each case as a lower limit; in case of a discrepancy 
between the two formulas (which usually will be 
insignificant with well-proportioned castings) it is 

f > » 

H -w 

* —^ 




Fig. 15-16. Dimensions of double web beams. 


1 u ' 

Fig. 15-17. Examples of beams with panels, a. I type; 
b. Channel type. 

safer to use the higher value. The formulas are 
entirely empirical and cannot be proved mathe- 
matically; experience has shown that their results 
are satisfactory to the foundry and make a casting 
of well-balanced dimensions, which usually also 
satisfies the static conditions except, perhaps, in 
extreme cases. 

The condition of uniform shrinkage means, theo- 
retically, uniform thicknesses; in practice, it means 
an upper limit to the thickness ratio between 
adjoining sections. A good limiting value for this 
ratio is 2 to 1, normally somewhat less. It is actually 
not desirable always to strive for completely 
uniform thickness; those parts of the casting that 
are exposed to more effective heat loss to the mold 
and which, therefore, would tend to cool faster, 
should be heavier than those parts where the cooling 
is slower; the end result is a casting with uniform 
shrinkage and low residual stresses. 

Corners should be rounded; this is imperative for 
internal corners, to avoid cracking; it is of lesser 
importance for external corners. The corner radius 
r, can be related to wall thickness t, as follows: 

For internal corners 
For external corners 

r = 0.5 t to 1,0 f 
r = 0A8 t to 0.2 f 

These are minimum values. However, radii in 
internal corners should not be uncritically increased, 
particularly not at places where ribs and walls cross 
or join, to avoid unnecessary accumulation of metal 
which would cause "hot spots," i.e., slow-cooling 
areas which invariably collect slag and develop 

While the principal dimensions of a cast fixture 
can be determined or confirmed by calculation, as 
explained in Appendix III, many details are not 

Ch. 15 



Fig. 15-18. {Left} A weak lug design. (Right) A strong 
lug design. 

amenable to such analysis, and will have to be 
designed by a combination of experience and "feel" 
on the part of the designer. 

Lugs for hold-down bolts should be provided with 
prongs of generous width because they may be 
accidentally stressed far beyond the limits anticipa- 
ted in any calculation. The same applies to ribs; 
in particular, ribs on brackets and other projecting 
parts with a large overhang. Examples are seen in 
Figs. 15-18 and 15-19. 

Fig. 15-19. 

(Left) A weak bracket design, 
strong bracket design. 

(Right) A 

Bends in thin walls should not show sharp corners, 
neither externally nor internally; in fact, they 
should be given generous radii of curvature, 
exceeding those quoted previously, as shown in Fig. 

stresses is obtained. These stresses consist of the 
remaining stresses originally in the casting, plus new 
stresses set up by the action of the cutting tool. Any 
casting for a .precision fixture must therefore be 
given a stabilizing treatment, either a normalizing, 
an anneal, or at least a stress relief. It is good 
practice, but not widely used, to sandblast and paint 
castings after this treatment to remove scale, oxide, 
and any remaining sand from mold and cores. 

Welded Fixtures 

Welded fixtures are, with few exceptions, made 
from low-carbon, hot-rolled steel assembled by 
electric-arc welding. This construction principle puts 
few restraints on the designer. There are virtually no 
thickness limitations, neither down nor up; metal of 
any thickness large enough to be used in a fixture 
can also be welded, and the proficient welder can 
deal with any special problem by, e.g., preheating 
prior to welding, and selecting a proper welding 
sequence to prevent heat accumulation, to cope 
with heavy sections or sections of widely differing 

Every known type of weld joint may be used; 
most frequently employed are those shown in Fig. 
15-21. Chamfered corners, and V- and U- joints are 
used less in fixture welding than in other structural 
welding, since fixtures are usually designed with 
generous dimensions so that fatigue is not con- 
sidered a serious hazard. Full penetration is, there- 
fore, not necessarily a requirement in joints between 
heavy sections. 


Fig. 15-20. Large radii of curvature required in thin-wall 

Effect of Machining on Castings 

As a general rule, machined surfaces should be as 
small as possible, partly because of the cost of 
machining, but also because the skin of the casting 
is of a particular structural value. The cast iron 
immediately below the skin is strongest, and this 
strength diminishes gradually towards the center of 
the section. 

When unstabilized castings are machined they are 
apt to distort as a result of the removal of metal in 
which the stresses were previously balanced against 
the stresses in the remaining metal. The casting 
distorts (warps) until a new balance of residual 

Fig. 15-21 . Types of welded joints used in fixtures. 

Weld Details 

A collection of some typical weld details for 
fixtures is shown in Fig. 15-22, which also gives 




Ch. 15 



Wr A * 


>4 or B 









Diagram a. 

7/16- 1/2 

11 -13 

V4 - 3/8 

11 - 13 

5/32 1 4 
3/16 5 

Diagram b. 

1 1/16 up 


26 up 






Diagrams c, d, and e. 

7/16- 1 
1 1/16 up 

11 -25 
26 up 







Fig. 1 5-22. Typical weld details and relative weld dimen- 
sions. Fillet sizes are determined by the thinner 
of the two adjoining sections. Where extra 
strength is required, use heavier fillets. The 
allowable stress under shocks is 5000 pounds 
per square inch (34.5 N per mm 2 ). 

Relative weld dimensions. A few typical fixture 
details are shown in Figs. 15-23 and 15-24; a 
composite fixture structure is shown in Fig. 1 5-25. 
Small projections for bosses and pads can be made 


Fig. 15-23. A welded U-shapc. 

Fig. 15-24. A welded bracket. 

1 ' 

Fig. 15-25. A composite welded fixture structure. 

by building up welding material and subsequent 
machining o.f the surface; examples are shown in 
Fig. 15-26. 

Components for welding should be precut as far 
as possible by sawing, milling, shearing (small thick- 
nesses only), and torch cutting. Large openings 



C I ?"'^ 

Fig. 15-26. Welded pads. 

Ch. 15 



C ) 

u u 


Fig. 15-27. A welded strap clamp. 

in plates are precut. Many contoured components 
are cut or rough machined before they are welded. 
Examples are the three-point clamp in Fig. 15-27, 
three different types of hinges in Fig. 15-28, and the 
built-up T-slot in Fig. 15-29. 

and gussets; two reinforcing components which 
are inexpensive in their application. Assume, for 
example, that a U-shaped open box, such as that 
shown in Fig. 15-23, Sacks rigidity; this deficiency is 
easily eliminated by adding two straps as shown in 
Fig. 15-30, or four gusset plates, as shown in Fig. 





r 1 


Fig. 15-30. The use of straps for increased rigidity. 

\ pt — ^~ — A 


Fig. 15-28. Welded hinges. 

Fig. 15-29. A welded T-slot. 

Design Rules 

Welded design differs from cast design in one 
important limitation: Curved shapes should be 
avoided. Straight plates, strips, and bars are cheap; 
and except with thin sections of no interest for 
fixture design, bending involves a serious cost 
increase. Welded fixtures, therefore, often will lack 
the more artistic look associated with cast shapes. 
Appearance, however, is a matter of taste and 
fashion, not of economy and efficiency. 

A seemingly small detail, actually a great asset 
to welding by increasing rigidity, is the use of straps 

15-31. On brackets and shelves, the use of a simple 
diagonal strap, as shown in Fig. 15-32, may be 
cheaper and even more efficient than a gusset. 

In addition to flat plates, the welded design makes 
extensive use of standard rolled structural shapes. 
The use of flats, with and without full corners, saves 
a considerable amount of cutting; the same is the 
case with square sections. Round sections are used 
in short lengths for bosses. Angles are widely used 



Fig. 15-31 The use of gusset plates for increased rigidity. 



Ch. 15 

Fig. 15-32. (left) A bracket with a gusset; .and (Right) 
with a diagonal strap. 

and are available in a great variety of dimensions, 
including large thicknesses, The other standard 
structural shapes, the channel and the I- and 
Z-beam, have relatively small wall-thicknesses and 
are seldom used, except in special cases, such as foT 
large T-sIot bases, as shown in Fig. 15-33, Typical 
examples of combinations of plates and standard 
sections are shown in the following illustrations. A 
drill jig with angle legs and an extensive use of flat 
sections, a design typical of many box-type jigs, is 
shown in Fig. 15-34. The same is the case with the 
drill jig shown in Fig. 15-35, which is built from 
plates and fiats, with short lengths of T-sections for 




Fig. 15-33. A welded T-slot base. 

There is no upper limit for the size of welded 
fixtures. For very large welded fixtures used in the 
aerospace industries, tube sections of medium wall 
thickness are virtually indispensible. The material 
may be of steel or aluminum. Circular as well as 
square tubes are used, and although square tubes 
are easier to cut and fit, they are slightly less 
economical with respect to material, in relation to 
strength and rigidity, and are not available in such 
large sizes as are circular tubes. 

To eliminate the danger of later distortion, welded 
fixtures should be annealed or normalized, then 
sandblasted and painted before machining. 

Within certain limits, welding permits the joining 
of steels of different hardness within the same 

Fig. 1 5-34. A welded drill jig with angle legs. 

structure. Jig feet can be made of low-grade tool 
steel, heat treated, and then welded lo the main 
structure. During annealing or normalizing, the 
material in the feet will be drawn and the resulting 
hardness will be approximately 35 Rockwell C, 
sufficient to provide good wear-resistance and still 
be machinable. 

Welded fixtures should be designed to have 
minimum surface areas for machining. In this 
respect, the designer has a little more freedom than 
with cast fixtures where certain compromises may 
have to be accepted for the sake of simplicity in the 
pattern design. He must visualize all machining 
operations and be certain that they do not also 
remove his weld beads. Unbelievable as it may seem, 
this sometimes happens! 

Fig. 15-35. A welded drill jig made of platus and flats. 

Ch. 15 



Comparison and Conclusions 

Some general rules can be laid down for the areas 
of application and relative merits of the three types 
of fixtures: 

The built-up fixture, including those carved out of 
one piece, is preferred for small parts in general; and 
for medium-size parts where the shape is simple; 
when welding and foundry facilities are not avail- 
able; or when delivery time is critical. It may 
take advantage of available standard sections, in the 
same manner as the welded fixture. It can be 
disassembled and changed, or its components may 
be reused in other fixtures. The cast fixture can, in 
principle, be designed for any desired size of part; 
however, its natural area is the medium size. It 
allows great freedom to the designer, lends to be 
heavy, requires access to a foundry, and involves the 
time and cost of pattern making, which may be 
substantial. On the other hand, the cast fixture may 
be economically superior if more than one casting 
from the same pattern is needed. When a larger 
number of nearly equal fixtures are needed, it may 
be economical to design and stock a supply of 
standard cast shapes, notably thick-walled channels 
and angles. However, it is not practical to attempt 
to change a cast fixture. 

The welded type of fixture has now become the 
most widely used. It is extensively covered in the 
literature and its superiority, relative to cast fix- 
tures, has been widely expressed; some of the state- 
ments made are quite correct, while others overly 
generalize; although they may signify the trend, in 
specific cases they do not always hold true. 

The welded fixture can be made lighter in weight 
than the cast fixture without sacrificing strength 
and rigidity. Procurement time can be less for a 
weldment than for a casting because of the time 
required for pattern making. It has been said that it 
takes less time and money to make a complete 
weldment than to make the pattern for it. It is 
probably nearer the truth to say that these two 
items are, generally, of the same order of magni- 
tude. The foundry trade is full of pitfalls, and 
perhaps il takes less experience and skill to design a 
welded structure than a successful cast structure. 

Areas requiring machining occasionally may be 
kept smaller in the welded fixture; it is also claimed 
that a weldment requires less additional thick- 
ness for machining allowance; this is, after all, a 
consequence of the amount of distortion to be 
expected. One literature source suggests machining 
allowances of: 1/8 inch (3 mm) on medium-size and 
1/4 inch (6 mm) on large-size welded fixtures. To 

this could be added 1/16 inch (1.5 mm) for fixtures 
consisting mainly of solid blocks with little welding. 
These figures are actually well in line with general 
practice for machining allowances for castings. 
Machining of steel takes about 5 percent more time 
than machining of cast iron for the same quantity of 
metal removed. On the assumption that weldments 
do require less machining than castings, it is also 
claimed that total machining cost will be 10 percent 
less, and, in conclusion, that the complete welded 
and machined fixture will result in a saving of 25 
percent over the total cost of the cast fixture. 

A welded fixture can, in principle, be changed by 
removing and adding components. This should also 
be taken with some reservation, because the altered 
fixture may well require an additional anneal and 
renewed machining. Rules such as these should be 
taken as guidelines only; sometimes they apply, 
sometimes they do not. 

The design of a fixture is not necessarily confined 
to any single type of construction alone. A com- 
bined construction may well present the most ad- 
vantageous solution. The frame or body can be 
welded or cast, and the precise locating surfaces can 
be screwed and doweled onto the frame. Thus, 
much, but not all, of the precision machine work 
can be done on the loose parts before they are 
attached to the body; often under more favorable 
conditions. Also, repair or alteration work on the 
fixture is more easily accomplished. 

Solid Fixtures 

Solid fixtures means those that are machined out 
of one piece of material. Simple jig plates would 
often fall into this category. The material is machine 
steel or light metal tooling plate. If hardened 
surfaces are required, the entire fixture is made of 
tool steel, A practical upper limit for fixtures made 
of machine steel is approximately 2 by 3 by 6 
inches (50 X 75 X 150 mm). The limit is not an 
absolute one, but increases with increased capacity 
and efficiency of the machining facilities available. 

A simple example of a solid fixture is shown in 
Fig. 15-36; 1 a drill jig for drilling, countersinking, 
and tapping the hole in a split shaft collar. The jig 
body is milled square and bored out, and a drill 
bushing, a handle, and a locator for the split in the 
collar are installed. The bushing is a slip bushing to 
allow for countersinking and is beveled to 
approximately fit the curved surface of the collar. 

! E, Thaulow, Maskinarkejde (Copenhagen: G.E.C. Gad's 
l'orlag, 1930) vol, II. 



Ch. 15 

Courtesy of S. Thaulow 
Fig. 15-36. A "solid" drill jig. 

A different type of drill jig is shown in Fig. 
1 5-3 7. 2 The part is a bearing bushing; the jig body is 
a pot with a handle. The bushing plate has a long 
stem terminating with a screw thread and the large 
nut that locks the bushing plate also provides the 
base on which the jig is supported during drilling. 
Almost all machining operations on the jig are 
performed in a lathe. An additional example of a 
solid fixture is shown later in Fig. 18-39. 

Courtesy ofE. Thaulow 
Fig. 15-37. A drill jig with three "soiid" components. 


Plastic Fixtures 

Plastic tooling is essentially made by casting or 
by laminating. The strength of these materials is, at 
best, comparable to the strength of cast iron, but 
often it is less. For this reason, plastic tooling 
materials are used only Tot tooling that is not 
exposed to heavy loads. Their principal areas of 
application are drill jigs, routing fixtures, inspection 
fixtures, and various types of templates. Within 
their natural areas of application they do have 
several advantages. They are light and are easy to 
handle. Since they are fabricated (cast or laminated) 
directly from the part, they lend themselves to 
forms with complex contours. They closely re- 
produce the contour of the master without com- 
plicated and costly machining. Tool details such 
as bushings and liners, are either embedded in the 
fixture body as it is fabricated, or potted in place 
after the plastic has cured. They require less lead 
time than metal fixtures and are also less expensive. 

If plastic fixtures are damaged they can be 
repaired, and if broken beyond repair, they can be 
replaced at a moderate cost. Design changes can 
be quickly and inexpensively incorporated in the 
fixture. Contours can be altered and detail parts 
added, deleted, or relocated without costly time 
delays or expensive machining. They are, therefore, 
applicable to prototype development. The low cost 
makes it economically feasible to use plastic fixtures 
for even short- production runs. For their ap- 
plication to production in large quantities, they 
have the advantage that they can be duplicated at a 
low unit cost and with precise accuracy. Technical 
details about plastic fixtures are presented in 
Chapters 3 and 14. 



Drawings, Dimensions, and Tolerances 

Economical Design 

Design and drawing costs represent a relatively 
large share of the total fixture cost since usually 
only one fixture is made from a set of drawings. 
A few shortcuts can be applied to reduce the cost 
of the drawings, but their integrity with respect 
to accuracy, completeness, readability, and clarity 
cannot be sacrificed. The fixture drawings are 
successively used by the checker, the planner, the 
production shops, and the inspection department 
and must quickly convey to each, the construction 
as well as the intended operation of the fixture. 
Overloaded drawings will require excessive time for 
study and deciphering, ambiguities generate requests 
for clarification, and inaccurate or incorrect draw- 
ings result in rejections, rework, and much loss of 
material and time. 

Economical design requires systematic work. The 
first step is to acquire the detailed and complete 
part drawing; the next step is to accumulate addi- 
tional pertinent information relating to the machine 
tool to be used and to the available accessories and 
general-purpose work holders. AH fixtures required 
for the complete machining program for the part 
are first laid out in sketches and reviewed together, 
a procedure that quite frequently results in useful 
modifications of the initial program, sometimes even 
in a revision of the design of the part. With these 
steps finalized, the shop drawings of all the fixtures 
can be prepared, lessening the risk of unpleasant 

Fixture Drawing Practices 

The layout drawing shows the complete fixture 
with the part located for machining, including an 
outline of the raw material. In this way, the ma- 
chining allowance on each machined surface is 

clearly recognized as is the clearance around the 
part. Adjoining details of the machine tool (table 
with T -slots, lathe spindle nose, etc.) are also shown. 
The assembly drawing is prepared from the layout 
and shows the fixture as seen from the operating 
side. It is also useful to indicate the direction of 
motion of the cutting tools, the direction of rota- 
tion of milling cutters, etc. Wherever possible, the 
details, as well as the assembly, are drawn to full 
size. Standard parts are not drawn in detail, but 
are shown and listed in the assembly. 

Drawings are made in pencil, with strong black 
lines to ensure blueprints of maximum contrast. 
A blueprint is exposed to rough treatment in the 
shop and its readability deteriorates rapidly from 
wear, repeated folding, and dirt. The part outline 
is drawn in phantom red lines. They are highly 
conspicuous, not only on the original, but also on 
blueprints where they show as ghost lines. 

General drafting practice, as used for product de- 
sign drawings, is followed with respect to pro- 
jections, sections, symbols, lettering, title block, ma- 
terial identification, heat treatment, surface rough- 
ness, standard tolerances, etc. The parts numbering 
system used for tooling detail parts is different than 
the system used for product detail parts. Sections 
are used generously as they are more informative 
than projected views. Sections arc cross-hatched 
for easy recognition in the drawing. The viewing di- 
rection on sections follows established practice or 
is conspicuously identified by arrows. 

A fixture drawing provides some specific informa- 
tion about the use of the fixture. It lists the number 
and type of cutters required for the operation. 
Where one fixture serves more than one operation 
and the part is differently located within the fixture 
for each operation, each location of the part is 
shown or clearly indicated. The same is done when 
a fixture is used for more than one part. 




Ch. 16 

Tricks of the Trade 

Templates for all commercial fixture components 
are available from the manufacturers and are used at 
the design stage as well as in the completion of the 
final drawings. 

To improve the contrast of blueprints, dimensions; 
dimension lines; arrowheads; and extension lines 
(witness lines) are drawn in India-ink. The addi- 
tional time required is negligible and the result is 
well worth the effort. 

In some aircraft companies, the very large fixtures 
for welding and assembly are built only from sketches 
of the main structure together with the detail draw- 
ings of the parts to be welded or assembled. 

in layout and assembly drawings it may some- 
times happen that one component, or the work- 
piece will obscure another component because their 
lines coincide. In this case, if one component is de- 
liberately drawn with a slight distortion, i.e,, a 
trifle longer or shorter, or to a slightly different 
scale, the confusion of lines is eliminated and the 
obscured part will then be exposed to view. The 
drawing gains in clarity, and the possibility of mis- 
interpretation and error is prevented by adding one 
or more dimensions to the distorted part. 

Placing the drawing paper diagonally on the board 
has several advantages not generally recognized. The 
lower left-hand corner of the sheet is held near the 
front edge of the board, the right-hand edge is raised 
20 degrees, and the corners are then fastened. The 
drafting machine is adjusted to work at this angle. 
Work is done faster as shadows are eliminated along 
straight edges, and it is easier to see the pencil point. 
Lettering is more easily done because of the slant 
of the drawing. The paper stays cleaner and the 
front edge does not become worn and tom, since 
the draftsman does not have to lean across the 

Dimensions and Tolerances 

Assembly drawings show the dimensions required 
for the inspection of the fixture; they are the di- 
mensions to the locating and clamping surfaces, to 
the tool guides (including drill bushing centers), and 
those relative to the attachment of the fixture to the 
machine tool. Every part contains one or more 
critical dimensions, that are recognized and identified 
at the beginning of the dimensioning and toleranc- 
ing procedure. Critical dimensions are those that 
are significant for the function of the part or for 
its compatibility with other parts in the fixture. 
Typical critical dimensions are hole center distances, 

the distance from a hole center to a machined sur- 
face, or the distance between two machined surfaces. 
A distance from a machined surface (or a hole 
center) to an unmachined surface is seldom critical, 
nor is the distance between two unmachined surfaces. 

Detail drawings of the fixture parts show critical 
dimensions, overall dimensions, and all others that 
define machining operations, Stock dimensions are 
listed in the bill of materials and are not needed on 
the detail drawings. 

In the conventional dimensioning system, all co- 
linear dimensions are written as a long chain that 
reaches from one end of the part to the other. 
The advantage here is that it provides a good check 
on the nominal numerical values as all the single 
dimensions in the chain must add up to the overall 
length of the part. However, it also has a disadvant- 
age in that the individual tolerances within the chain 
add or subtract in an unpredictable manner. If, for 
example, all tolerances come out with their plus 
maximum values, it would require an excessively 
large tolerance on the overall length of the part. 

The conventional dimensioning system is now 
supplemented, and is gradually being replaced, by a 
new system, the "coordinate dimensioning system," 
or the "coordinate system," Modern manufacturing 
practices, as exemplified by the use of jig borers, 
jig grinders, and N/C (Afumerically Controlled) ma- 
chine tools, require each dimension to be defined as 
the distance between the particular point or sur- 
face and a common datum or reference line, point, 
or plane. Fundamentally, two perpendicular refer- 
ence lines are required for dimensions in one plane. 
Three reference planes are required for complete 
definition of all dimensions on a three-dimensional 
part. Where more than two reference lines or points 
are needed in one plane, their relative location must 
be clearly defined. A set of two reference lines is 
equivalent to the x and y axes used in analytical 

In a jig borer or jig grinder, the table with the work 
can be moved relative to the spindle in two perpendi- 
cular directions (the coordinate axis directions), as 
shown in Fig. 16-1, and the table settings (the x 
and y coordinates) can be read out with the ac- 
curacy of 0.0001 inch (0.003 mm). In most ma- 
chines, the readings axe direct, when the table moves 
toward the left and toward the column of the ma- 
chine (away from the operator), which again corre- 
sponds to an apparent or relative movement of the 
spindle to the right (the x axis direction), and 
toward the operator (the y axis direction). For 
convenience in the operations and as a safety mea- 
sure to avoid errors, the dimensions shown on the 

Ch. 16 



movement away 
from column 







Fig. 16-1. Movements and positions of the jig borer table. A. Position of table relative to spindle before table movement; 
B. Position of table after moving in conventional direction by direct reading of coordinate measuring system. 
Note that the "effective" movement of the jig borer spindle is opposite in direction from the actual table 

part drawing should also go from left to right and 
from top to bottom, and the two reference tines in 
the plan view should originate from a point at, or 
near, the upper left corner of the part. 

Numerically controlled machine tools are designed 
somewhat differently. The reference axes on the 

drawings should appear in the lower left-hand corner; 
Le., the dimensions go left to right and from bottom 
to top. However, tool drawings should be dimen- 
sioned as shown in Fig. 16-2, since tooling compon- 
ents are normally machined on jig borers, not on 
N/C machines. 



Ch. 16 

J — f 






Fig. 16-2. Dimensioning from two reference lines. 

Dimensioning of hole centers from two reference 
lines (coordinate axes) is shown in Fig. 16-2. In the 
upper view, dimensions are written on ordinary 
dimension lines with arrowheads and leader lines to 
the corresponding points in the drawing. The ar- 
rangement of the dimension lines shows dearly 
where the reference lines are assumed to be. Direct 
center-to-center dimensions, such as a in the illustra- 
tion, are not part of the system, but are calculated 
and entered on the drawing, labeled "REF," and are 
to be used for inspection. When the number of holes 
is large, some drafting and lettering work is saved by 
simply numbering the holes on the drawing and 
tabulating them by number, diameter, and x and y 

coordinates. In this case it is necessary to define the 
coordinate axes on the drawing. 

A different method of dimensioning from reference 
lines is shown in the lower view of Fig. 16-2. The 
reference lines are defined, and the dimension to 
each significant point is written on the leader line. 
This method is frequently used, not only because it 
simplifies the lettering work, but because if also 
makes it much easier to read the numbers. 

With the coordinate dimensioning system, each 
single dimension and tolerance is now independent 
of all others. Sometimes, it is necessary to combine 
two or more dimensions in a short chain. This is 
permissible, provided the individual tolerances are 

Ch. 16 



selected so that the total tolerance on the chain does 
not conflict with other tolerances within the part. 
The transfer of dimensions with or without toler- 
ances from the conventional system to the coordin- 
ate system is governed by the following two prin- 

1. For wntoleranced dimensions, the difference of 
any pair of dimensions (coordinates) on the co- 
ordinate system must equal the dimension that they 
replace on the conventional system. 

2, For toleranced dimensions, the sum of the 
tolerances of any pair of dimensions on the coordin- 
ate system must not exceed the tolerance of the 
dimension that they replace on the conventional 
system. The procedure is explained in full detail in 
Appendix II at the back of the book. 

Fixture Tolerances 

The accuracy of a machined part is less than the 
inherent accuracy of the machine tool and the fix- 
ture by means of which the part was made. This is 
known as the "degeneration of accuracy." To ensure 
interchangeability of the machined parts, it is there- 
fore necessary to prescribe closer tolerances of the 
dimensions of the fixture than of the dimensions of 
the part. The crucial question is what tolerances 
shall be applied to the fixture dimensions to ensure 
correct tolerances on the part dimensions. The 
literature is generous with suggestions. They range 
from one half to one tenth of the part tolerances. 
Each recommendation is as good as any other re- 
commendation, and none of them will guarantee the 
correct result. 

Fixture tolerances are determined by a combina- 
tion of common sense, practical judgment, logical 
conclusions, analysis, and calculations, usually of an 
elementary, sometimes trivial nature. The pro- 
cedure can start with the part tolerances and 
work through to the fixture tolerances, or it can 
start with a set of assumed or selected fixture toler- 
ances, calculating the resulting part tolerances, and 
comparing these with the required tolerances from 
the part drawing. 

Close tolerances are expensive, thus tolerances are 
always selected as wide as possible, consistent with 
the proper functions of the part. A part that func- 
tions with wide tolerances will also function with 
close tolerances, but not necessarily vice versa. 
An initially selected tolerance that is found to be 
too wide can be reduced without risk. In the search 
for fixture tolerances it is therefore recommended to 
start with those dimensions that offer the best 
possibility of using wide tolerances. 

Tolerances are needed on dimensions relating to 
the following fixture elements: 

1. Part locators and cutter positioners 

2. Devices for attaching the fixture to the machine 
tool (positioning keys, slots for clamping bolts, etc.), 
for attaching gages to the fixture, and surfaces for 
the installation of standard components and inter- 
changeable or replaceable fixture parts (bushings, 
inserts, etc.). 

Example— Two 0.1 9 1-inch- diameter holes are to be 
drilled in the round disc by means of the drill jig 
shown in Fig. 16-3. The part is located by its 
periphery in the inverted nest in the upper part of 
the jig. There is no clamping device, but when the 
first hole is drilled, a pin is inserted to lock the part 
in position relative to the jig. The two hole location 
dimensions are apparently critical dimensions. The 
most liberal tolerance is the .004 inch on the loca- 
tion of the hole to the left. If there were no other 
considerations this would permit the center of the 
part to move a total of .004 inch relative to the jig, 
and a part with the maximum diameter of 1.4375 
inches could, so far, accept a nest diameter of 1.437 5 
+ .004 = 1.4415 inches. This dimension, however, 
is not acceptable because it does not include any 
tolerance, it does not allow for wear, and it does not 
recognize the existence of parts with less than 
maximum diameter. 

A part of this small size requires a minimum nomi- 
nal clearance of .0005 inch to enter the nest which 
gives a minimum nest diameter of 1 .4375 + .0005 = 
1.438(0) inches. To this is applied a manufacturing 
tolerance (machining tolerance, "toolmaker's toler- 
ance") of .001 inch resulting in the nest diameter 

inches. If the nest is at its maximum of 

1.439 it provides a clearance of .0025 inch against 
a part of minimum diameter 1.4365 inches. These 
relationships are shown in Fig, 16-4, The selection 
of the fixture dimension and tolerance is in accord- 
ance with sound economic principles. The cost is 
determined by the tolerance .001 inch, which is 
reasonable for this class of work, and not by the 
resulting minimum clearance of .0005 inch which is 
in a sense, incidental only, since it depends on the 
fixture and the selected part, not on the fixture as 
such. The resulting maximum clearance is within 
the maximum permissible value of .004 and also 
allows an ample margin. 

As a part of minimum diameter is shifted through 
the maximum clearance of .0025 inch, the center of 
the part is shifted between positions, .00125 inch on 



Ch. 16 

DRILL ,191 
(= 11 DRILL) 

3 f 
% ±.005 













Fig. 16-3. A part with dimensions and tolerances, and its drill jig. 

each side of the center of the nest. Let a be the 
distance from the center of the nest to the center of 
the drill bushing, first assumed to be without toler- 
ance. Then, to satisfy the part tolerances, we have 

a -.00125 > .498 and a + .00125 < .502 
.49925 < a <. 50075 

Like any other dimension, a requires a tooimaker's 
tolerance which again, is selected as .001 inch. 
There must also be an allowance of .00025 inch for 

wear in the bushing (actually also including the in- 
itial clearance between the drill and the bushing). 
These requirements are satisfied by selecting the 

center distance as ' 4Qt) c btct, which provides the 

required wear allowance of .00025 inch and leaves 
an unclaimed margin of safety of .00025 inch. 
The maximum tolerance on the hole center dis- 
tance on the part is .002 inch; with a tooimaker's 
tolerance of .001 inch on the bushing center distance 
in the jig, this requirement is comfortably satisfied. 









,00125— * 
Fig. 16-4, Resulting clearances for the part shown in Fig. 16-3. 

Ch, 16 



It is usual practice in such cases to split the part 
tolerance. This is done by selecting the fixture dim- 
ension as 'qqqj- inches, which provides an allowance 

of .0005 inch on each side. This allows the 0.00025 
inch for bushing wear and still leaves an unclaimed 
margin of safety. 

Toolroom Tolerances 

The toolmaker's tolerance of .001 inch (0.03 mm) 
can be maintained economically in ordinary tool- 
room operations on conventional lathes, milling 
machines, and grinders. Precision grinders with 
hydraulic feed and positioning devices work to toler- 
ances of less than .0005 inch (0.013 mm). With the 
addition of a high precision gage, tolerances of ,0001 
to .0002 inch (0.003 to 0.005 mm) can be main- 
tained consistently. The jig borer maintains .0005 
inch (0.013 mm) under average conditions, and this 
can be reduced to .0002 to .0001 inch (0.005 to 
0.003 mm) by careful work under the best condi- 

Example: A crank arm as shown in Fig, 16-5, is to 
be straddle milled on the two opposite sides of the 

7 50 

crank arm boss to a width of "743 inch. One side of 

the crank arm boss is to be aligned with the side of 
the center boss (which is already machined) within 
±.002 inch. The milling cutters are positioned with 
a feeler gage from a setting block which is located 
from the same locating surface in the fixture as the 
center boss. 

1 -"a 1 



I r ig. 16-5. Tolerances for setting block and milling cutters. 

The cutters are mounted on an arbor. The space 
between the cutters is checked after each regrinding 

and is maintained at - 750 inch. With a mean cutter 

space of ,749 inch and a mean thickness of the 

feeler gage of ,120 inch, the mean width of the 
setting block is 

.749-2 X .120 = .509 inch 

The mean width of the shoulder on the setting block 
is .120 inch, the same as the thickness of the feeler 
gage. The toolmaker's tolerances are .0004 inch 
on the feeler gage, .0006 inch on the setting block, 
and .001 inch on the shoulder. The actual dimen- 

I 1 QQ 

sions, are, on the feeler gage ' ( j n ^ inch, on the set- 

._, , -5093 . . . " . ,. .1205 

tmg block, 5Q07 inch, and on the shoulder 1 jq5 


When the right cutter is positioned from the right 

side of the setting block, the extreme alignments 

with the locating surface (which must be within 

+ 002 inch) are 



shoulder, minimum -.1195 maximum -.1205 
feeler gage, maximum +.1202 minimum + .1198 
alignment + .0007 - .0007 

Plus and minus alignment means that the cutter is 
positioned to the right and left of the locating 

When the left cutter is positioned from the left 
side of the setting block, the extreme alignments for 
the right cutter are 

Inch Inch 

cutter space, minimum + .748 maximum + .750 
shoulder, maximum -.1205 minimum -.1195 

setting block, maximum - .5093 minimum - .5087 
feeler gage, maximum - . 1 20 2 minimum - .1 198 
alignment - .002 + .002 

which is still permissible. 

Cal louts 

Additional instruction for the machining of the 
part is provided by callouts. Machining operations 
that are obvious are not called out, but callouts are 
used where a sequence of several operations is re- 
quired at one location, such as for a hole, and where 
an operation must be performed in accordance with 
specific requirements that are not conveniently de- 
scribed by conventional or coordinate dimensioning. 
Callouts are short, often one word only, and even 
that may be abbreviated, or they are formulated in a 
sort of telegram style when more than one word is 
needed. Some companies forbid the use of callouts 
because they prefer to have decisions regarding 




Ch. 16 




CARB HDN & GR (carburize, harden and grind) 

DRILL .50 DIA 1.25 DEEP 


.75 DRILL, .50 DEEP, 4 HOLES 




# 30 (.128) DRILL 2 PLACES 





HT TO 30-32 RC 

















C'S'K (countersink) 



.001 TIR (rota! /ndicator heading) 


FIR (Full/ndicator .Reading) 



LENGTH (used foi a conical taper) 


TAPER .1500 ± .0015 PER INCH 

PERP TO SURF (perpendicular to surface) 

(used for a flat taper) 



BOTH SIDES (used to save repeating a dimension) 

REAM 1.250 DIA 




TAP 1/2-20, 1 DEEP 

DIA (diameter) 

TAP 1/2-13 UNC-2B, 1 DEEP 

R (radius) 

1/2-13 NC THREAD 


1/2-13 NCTHD 

R SPHER (spherical radius) 

DRILL & TAP 1/2-13 UNC, 2 HOLES 180° APART 

BEND R 1/4 


TYP (typical) 


REF (reference) 


BSC (basic) 


SYM (symmetrical) 


4 (center line) 


MMC (Maximum A/ateriai Condition) 


DEG (degrees) 


GA (gage, indicating plate thickness, for example 


#12 GA = . 105 inch) 


4> (diameter) 


mm (millimeter) 


cm (centimeter) 

1/8 X 45° CHAMFER 

m (meter) 




A callout "WELD" foUowed by the weld dimension 


may be used. However, using the standardized symbols 


for welded joints is recommended. 

Ch. 16 



machining operations made by the workshop per- application. Common and typical callouts are 
sonnel. However, industrial callouts are widely used presented in the listing on the previous page, and 
and the tool designer should be familiar with their some typical operations are shown in Fig. 16-6. 

D F 

Fig. 16-6. Typical operations, frequently identified by callouts: A. Spot drill, center drill; B. Spot face; C. Back face; 
D. Counterbore; B. Countersink; F. Bore, 



Standard and Commercial Fixture Components 

Advantages of Standardization 

USA Standards 

Fixture components are, in general, small and are 
used in large quantities. Their design is closely de- 
termined by the function of the particular compon- 
ent, and no consideration of taste or style is involved. 
For these reasons, fixture components offer a wide 
field for standardization. 

Standardized components offer significant advant- 
ages to the user. "Design" is reduced to selection of 
a component of suitable size from a table, and actual 
design time is eliminated. The quantity of identical 
parts required is increased, and production cost is 
reduced. When the standardization process tran- 
scends the boundaries of individual firms, it opens 
the way for mass production of components by 
specialized manufacturers with further possibilities 
for cost reduction and quality improvement. High- 
precision and high-quality parts (small drill bushings, 
hardened and ground to close tolerances) are avail- 
able in today's market at prices from less than a 
dollar; this is less than the cost of pulling a vellum 
from the file and having a blueprint made! 


The drill bushing and fixture component industry 
in the USA is today a multimillion dollar industry 
and is steadily expanding. In I 958, the manufac- 
turers within this industry organized the N.i.J.F.C.M. 
(National Institute of Jig and Fixture Component 
Manufacturers, Oakland, California), A major ac- 
tivity of this organization is the standardization of 
fixture components. As standards are adopted, 
specifications are made available to its members for 
incorporation into their manufactured products and 
are presented to consumers through the members' 
marketing literature. 

Standardization of fixture components in the 
USA (including drill bushings which, traditionally 
are mentioned separately) is now found on three 
levels. National standardization started in 193 5 
with the issue of American Standard for Jig Bush- 
ings. This standard has been frequently revised; the 
current edition is ANSI B94.33-I 962 (R1971), Jig 
Bushings. It covers the types of bushings illustrated 
in Figs. 14-1 and 14-2 and described in Chapter 14. 

Proposed Standards 

The next level of standardization is a set of pro- 
posed standards prepared by the N.I.J.F.C.M. for 
submittal to the American National Standards 
Institute (ANSI); a package of individual standard 
proposals, covering most of the components needed 
for clamping and locating. With the unified recom- 
mendation of the industry concerned, it may be ex- 
pected that these proposals will be approved and de- 
signated as USA Standards in the near future. 

It should be noted thai this proposal will standard- 
ize not only sizes, dimensions, tolerances, and, in 
some cases, materials, but also part numbers, which 
when adopted, will greatly simplify specifying and 
ordering these items. 

Manufacturers' Standards 

The third level of standardization is at the manu- 
facturers" level. Much standardization and unifica- 
tion has been going on through the years, and many 
items have been virtually standardized on all signifi- 
cant dimensions. These are listed in individual 
manufacturers' catalogs which should be consulted 
for the items required. One area with the least 


Ch. 17 



standardization is the numbering system; but this 
condition will be simplified by the proposed stan- 
dards. As it now stands, each manufacturer has his 
own serial number system. Cross reference tables 
have been prepared by which interchangeable or, at 
least, equivalent parts from different sources can be 

Drafting templates arc also available for most com- 
mercial components. When used systematically, they 
can save much time in all phases of the design, from 
the initial layout to the final vellum. 

Proprietary and Patented Fixtures and Components 

Most of the devices and components described in 
this book are in the public domain and may be util- 
ized freely. However, several of the commercially 
available components and fixtures are covered by 
some degree of legal protection, usually in the form 
of one or several patents. Where information about 
proprietary rights, patents or otherwise, has been 
available, it is so indicated, either in the text or in 
captions to the illustrations. Any device and design 
so designated is protected and the fixture designer 
is advised not to copy it nor to utilize it in any 
other unauthorized manner. 

Commercial Fixture Components 

A review of the presently available components 
with evaluations, brief descriptions, and data for 
their range of sizes and capacities follows. Dimension 
symbols on line drawings will indicate to the fixture 
designer the dimensional information that is avail- 
able (from manufacturers' catalogs) about each com- 
ponent. Drill bushings are not included since they 
are discussed at considerable length in Chapter 14. 

Bolts, Screws, and Associated Parts (See Fig. 17-1 a 
through ii.) 

Studs (a) are made from high-strength steel and 
have a number of uses too diversified to be specified. 
They can be joined by coupling nuts (f) to create 
any required length. Threads are made to a nut fit 
on both ends. A secure fit at installation is obtained 
by the use of a bonding agent on the thread. 
Range: Thread from 1/4-20 to 1-8, length from 
1 1/2 to 12 inches. 

Eye bolts (b), jig latch bolts (c), and swing bolts 
(d), are used where the bolt must be swung out of 
place to allow a strap to be removed or a jig leaf to 
be opened. Another use is with cam clamps where 

it is desired that the cam rock back and forth for 
direct force application. Range; Thread from 1/4- 
20 to 3/4- 1 0, length from 2 to 6 inches. 

T-bolts (e) are used with related parts such as 
clamp straps, flange nuts (g), and spherical washers 
(1) to create various work-clamping arrangements. 
They are also used for clamping the fixture down to 
the machine tool table. These bolts are highly 
stressed in service and are made from heat treated 
alloy steel with 150,000 psi minimum tensile 
strength. Range: Thread from 1/2-13 to 1-8, length 
from 1 1/2 to 12 inches. 

Coupling nuts (f) are long nuts and are used to 
couple two studs to create a stud of desired length. 
They are also used where a nut of exceptional 
length is needed for other purposes. Range: Thread 
from 3/8-16 to 1-8, length from 1 to 2 1/2 inches. 
Flange nuts (g) have a large bearing area to ensure 
increased surface contact with a clamping compon- 
ent. For many purposes, combining a flange nut 
with a set of spherical washers (/) to eliminate un- 
equal load on the screw thread, caused by slight 
misalignment, is recommended. Range: Thread from 
1/4-20 lo 1 1/4-7 and from 1/4-28 to I 1/4-12. 
Spherical flange nuts (h) combined with bottom 
spherical washers (1) are used to compensate for 
minor irregularities between clamp strap and part 
being clamped, and to eliminate unequal thread 
loads caused by slight misalignment. Range: Thread 
from 1/4-20 to 1 1/4-7. 

Acorn nuts (see Fig. 17-3 e) are nuts that are 
closed at one end to protect screw thread against 
dirt and damage. Range: Thread from 1/4-20 to 

Knurled lock nuts (i) are used for quick thread 
locking. They can be tightened by hand or with a 
1/4-inch rod. Range: Thread from 3/8-16 to 

T-slot nuts (j) are used with studs for clamping a 
fixture down to the machine tool table. They are 
adapted to standard machine tool table T-slots. 
Two series are available: "standard" and "N. I. 1. F. 
C. M. standard." The screw thread in the nut is so 
designed and cut that the stud cannot be turned 
through the nut and down into the table. Range: 
Thread from 5/16-18 to 3/4-10. 

Interchangeable sine fixture keys (k) are keys for 
use in the machine tool table slots and permit the 
use of the fixture in slots of varying widths. They 
are inserted in the T-slot from above, rotated until 
they fit in the wide part of the slot, and locked into 
position in the fixture by a slight turn of the set- 
screw. Range: Width across flats (for entering the 
T-slot) from 1/2 to 1 1/8 inches. 



Ch. 17 

| |M H** *»!■ MM W l> (•* M»» H- m 






l — li|i|i|il l i| i |i|i l l \ 

T ^FIT ^ \ 



^^ «U«i 


- G 


- D — 1 


r— 1/8 r A 

\threao size 


— •- 


B - 


e a- 


H B K h— C 





h ^-a: j- 

L_D —J 

Fig. 17-1. *Bolts, screws, and associated parts, a. §Stud; b. ttEye bolt; c. + Jig latch bolt; d. f Swing bolt; e, tT-bolt; 
f, ttCoupling nut; g. tt Flange nut; h. § Spherical flange nut. 

illustrations courtesy of the following companies: ? American Drill Bushing Co.; ft Northwestern Tools, Inc.; § Morton 
Machine Works. 

Spherical washers (1) act as a ball-and-socket to 
compensate for any slight misalignment between 
clamp strap and part being clamped, and to eliminate 
undue stresses on threads. Hole size is 1/16 to 1/8 
inch larger than the corresponding stud to allow for 
equalizing action. Range: Corresponding stud dia- 
meter from 1/4 to 1 1/2 inches. 

C- washers (m) are for easy removal to speed-up 
clamping and release of the part. A wire hole is 
provided for attachment to the fixture. Range: 
Width of slot A from 9/32 to 1 1/32 inches. 

Connecting cables (n) are used for C- washers and 
other loose parts, for attachment to fixture. They 
are made of nylon covered stranded-steel cable in 

Ch. 17 






. TABLE , 
| SLOT 'I 






f— j-VI. 





Fig. 17-1 (Cant.). *Bolts, screws, and associated parts, i. t Knurled lock nut; j. t+T-slot nut; k. t Interchangeable sine 
fixture key (U.S. Pat. 2,707,419); l. § Spherical washer; m. §C-washer; n. t Connecting cable; o. tt Swing 

illustrations courtesy of the following companies: 'American Drill Bushing Co.; ''Northwestern Tools, Inc.; §Morton 
Machine Works. 

1 5-inch length and provided with ferrules for crimp- 

Swing C-washers (o) are for easy removal to speed 
up clamping and release of the part. Washer is held in 
place by shoulder screws (p or q). Range: Radius B 
from 1 to 1 3/4 inches, thread from 1/4-20 to 3/8-16. 

Slotted shoulder screws (p) provide a precision 
ground shoulder diameter for use as a pivot for a 

swinging or rotating component. Range; Thread 
from 10-32 to 3/8-16. 

Socket shoulder screws (q) provide a precision 
ground shoulder diameter for use as pivot pins for 
C-washers, swing clamp straps, and other rotating 
components. Screw head contains socket for 
socket hex wrench. Available as N, I. J. F. C. M. 
standard. Range: Thread from 10-24 to 5/8-11. 



Ch. 17 

F -# 



q c/ A 


+ .000 


' i ' r 

- r~~~ * 








Fig. 17-1 (Con!,). *Bolts, screws, and associated parts, p. § Slotted shoulder screw; q. f Socket shoulder screw; r. tHand 
knob and screw; s. tHand knob screw assembly; t. '''Knob swivel screw; a. **Knurled-head screw. 

Illustrations courtesy of the following companies: §Morton Machine Works; ^American Drill Bushing Co.: ** Monroe 
Engineering Products Inc. 






A 3 






Fig. 17-1 (Cont). * Bolts, screws, and associated parts, v. t Torque-he ad screw; w. 1'Quarter-turn screw; x. tHalf-turn screw; 

j . * Threaded adjustable locating button; z. tJack screw; a a. * Bar knob. 
'illustrations court ?sy of the following companies: American Drill Bushing Co.; **Monroe Engineering Products Inc. 

Hand knobs and screws (r) are used for hand- 
tightened holding and clamping functions. Hand 
knob is cadmiun plated cast iron; screw is heat 
treated steel witl black oxide finish. Range; Thread 
from 1/4-20 to 5/8-11, length from 1 3/4 to 3 

Hand knob screw assemblies (s) are used for hand- 
tightened holding and clamping functions. Relieved 

tip protects end threads from damage caused by 
slight peening. Range: Thread from 1/4-20 to 
5/8-11, length from 1 to 3 1/2 inches. 

Knob swivel screws (t) are used for ha rid- tightened 
holding and clamping functions. Swivel shoes pre- 
vent marring of finished surfaces of soft materials 
such as aluminum and copper. Shoe stops rotation 
immediately upon contact with workpiece, swivels 



Ch. 17 






7 ^O 




- — 8— 

— £ 

\ — - 

I k 

V c - 



' _| 











Fig. 17-1 fCont.}. *Bolts, screws, and associated parts, bb. T4-and5-prong hand knobs; cc. ttStar hand knob; dd. t Knurled 
knob; ee, 'Speed ball handle; ff. ttPlastic ball knob; gg. § Finger handle. 

Illustrations courtesy of the following companies: ^American Drill Bushing Co.; ^ Northwestern Tools, Inc.: ^Morton 
Machine Works. 

3 degrees in all directions to compensate for minor 
surface irregularities, pulls off easily and snaps on 
for installation in fixture mounting hole. Range: 
Thread from 1/4-20 to 3/4-10, length from 1 1/4 
to 4 13/16 inches. 

Knurled head screws (u) are used for light-duty 
holding and clamping applications. Knurled head 
provides for easy finger tightening. Relieved tip 
protects end threads from damage caused by slight 
peening. Threads are rolled, which provides increased 

Ch. 17 



Fig. 17-1 (Com,). *Bolts, screws, and associated parts, hh. tHand wheels /Right) with handle; ii, § Machine handle. 
Illustrations courtesy of the following companies; ' American Drill Bushing Co.; § Morton Machine Works. 

strength and wear resistance and a smoother surface. 
Range: Thread from 10-24 to 1/2-13, length from 1 
to 3 1/2 inches. 

Torque head screws (v) have a spring-loaded clutch 
which is built into the head and releases at a preset 
torque, In this way they prevent overclamping and 
distortion of the part. In some models the releasing 
torque can be adjusted from the outside. Torque 
screws are provided with check nuts and can be 
supplied with swivel pads and nylon tips. Range: 
Thread from 10-32 to 5/8-11, end force from 10 
pounds to 28 pounds for fixed torque types and from 
to SO pounds for adjustable torque types. 

Quarter-turn screws (w) are quick-locking fasten- 
ers for leaf jigs and jig plates. Corners are chamfered 
to provide self alignment of screw head with latch 
slot when closing the leaf. Range: Thread from 
10-32 to 1/2-13, length from 1 to 2 1/4 inches. 

Half-turn screws (x) are quick-locking fasteners for 
leaf jigs and jig-plates. Screw-shoulder locks and 
unlocks the plate by rotating the screw one-half 
turn. Range: Thread from 10-32 to 1/2-13, length 
from 1 to 1 1/4 inches. 

Threaded adjustable locating buttons (y) are used 
together or in combination with fixed locating 
buttons and pins to accurately locate workpiece in 
fixture. Range: Thread from 10-32 to 5/8-18, 
length from 1 to 3 inches. 

Jack screws (z) are used in fixtures to support 
irregularly shaped workpieces, such as castings, and 
to prevent elastic distortion (springing) of thin work- 
pieces during machining. Range: Thread from 
3/8-16 to5/8-Il, length B from 1 1 /4 to 2 1 /2 inches. 

Bar knobs (aa) are used in heavy-duty clamping 
applications. They are made from high-strength 
ductile iron and can be tightened by inserting a bar 

between the vertical prongs for maximum leverage. 
Knobs can be obtained as unmachined blanks or as 
finished knobs, with choice of tapped or reamed 
mounting hole. Range: Hole, blank, tapped from 
3/8-16 to 1-8, reamed from 3/8 inch. 

Four- and five-pronged hand knobs (bb) are cast 
from gray iron, tumbled smooth, and cadmium pla- 
ted. Faces of reamed or tapped knobs are machined 
square with hole. Four-pronged knobs- Range: Hole, 
blank, tapped from 10-32 to 5/8-11, reamed from 
3/16 to 5/8 inch, diameter A from 7/8 to 2 1/2 
inches. Five-pronged knobs- Range: Hole, blank, 
tapped from 5/8-11 to 3/4-10, reamed from 5/8 to 
3/4 inch, diameter 4, 3 inches. 

Star hand knobs (cc) are available in choice of 
aluminum or cadmium plated cast iron. Knobs can 
be obtained as unmachined blanks or as finished 
knobs with tapped hole. Range: Hole, blank, 
tapped from 1/4-20 to 5/8-11, diameter A from 
1 1/8 to 3 inches. 

Knurled knobs (dd) are suitable for adjustment, 
clamping and locating devices. Knurled headprovides 
nonslip finger grip. Knobs are made with tapped 
holes. Range: Thread from 10-24 to 3/4-10, 
diameter B from 3/4 to 2 1/2 inchesL 

Speed-ball handles (ee) are balanced to permit 
rapid spinning for quick clamping and release of 
the work. Wide handle permits greater leverage. 
Handles can be obtained as unmachined blanks or as 
finished handles with choice of tapped or reamed 
mounting hole. Range: Hole, blank, tapped from 
1/2-13 to 3/4-10, reamed from 1/2 to 3/4 inch, 
width A from 4 3/4 to 8 inches. 

Plastic and steel ball knobs. Lightweight plastic 
ball knobs (ff) provide a comfortable, rustproof 
handgrip for actuating levers. Plastic knobs, except 



Ch. 17 

the largest size, have threaded brass inserts. Steel 
knobs are recommended for use where added weight 
is desirable for easier lever actuation. They are 
available as unmachined blanks or as finished knobs 
with hole drilled and tapped, ready for mounting. 
Plastic knobs-Range: Thread from 10-32 to 5/8-18, 
diameter B from 1 to I 7/8 inches. Steel knobs- 
Range: Hole, blank, tapped from 3/8-16 to 5/8-18, 
diameter B from 1 1/2 to 2 inches. 

Finger handles (gg) are miniature handles to be 
used as finger grips for actuation, control, or adjust- 
ment of smaii movable parts. Range: Thread from 
10-24 to 5/16-18, length C from 11/ 1 6 to 1 1/4 

Handwheels fhh) are made from cast iron and can 
be obtained as blanks or finished machined with 
polished rims and unpolished surfaces painted. Small- 
est size hand wheel is solid, larger sizes have four 
spokes. Range: Diameter^ from 3 to 12 inches. 

Machine handles (ji) are used with handwheels and 
for various actuating and gripping purposes They 
are machined and polished to a smooth finish. 
Shape is designed for convenient grip, and speed and 
ease of operation. Solid handles can be obtained" 
with a press fit, or with a threaded shank, revolving 
handles are made with a press fit. Solid handles- 
Range: Shank diameter^ for press fit from 1/4 to 
1/2 inch, shank thread from i/4-20 to 1/2-13, 
length B from 1 23/32 to 5 inches. Revolving hand- 
les-Range: Shank diameter A for press fit from 
5/16 to 1/2 inch, length B from 2 5/8 to 5 1/8 

Quick-acting Screw Components (See Fig. 17- 2 a 
through d.) 

Single locking levers (a) provide a quick and 
efficient means of permanently locking and clamp- 
ing workpicces or movable fixture parts with fairly 
large clamping pressure. Range: Hole, blank, tapped 
from 3/8-16 to 5/8-1 1, reamed from 3/8 to 5/8 
inch, length of arm from 2 to 4 inches. 

Double locking levers (b) are used for quick and 
permanent locking with large clamping pressure. 
Range: Hole, blank, tapped from 3/8-16 to 5/8-1 1, 
reamed 5/8 inch, width over handles from 4 to 8 

Quick-locking levers (c) have a part of the screw 
thread removed to provide a smooth hole for fast 
removal from the stud when the handle is tilted. 
The lever locks or releases by rotating it one quarter 
turn. When in the unlocked position, the weight of 
the handle automatically puts the lever in the re- 
lease position for quick removal from the stud. 

Courtesy ofJergeiis Inc. 
Fig, 17-2, Quick-acting screw components, a. Single locking 
lever; b. Double locking lever; c. Quick lock 
lever; d. Quick lock knob. 

Range: Thread from 1/2-13 to 3/4-10, length of 
arm from 4 3/8 to 5 3/8 inches. 

Quick-locking knobs (d) have a part of the screw 
thread removed to provide a smooth hole for fast 
removal when the knob is tilted. The knob locks or 
releases by rotating it one quarter turn. Range: 
Thread from 1/4-20 to 5/8-11, diameter from 1 1/8 
to 3 inches. 

Screw Clamp Assemblies (See Fig. 17-3 a through 

Strap clamp assemblies (a through h) are suitable 
for a wide variety of clamping applications. They 

Ch. 17 









— I ; » { ] 1 


-j G h H — - 


■^3^1 *J** 



-C|rt»jrt — (J-H-hB 


^^^ ^-p 



— C 








Courtesy of American Drill Hushing Co. 

Fig. 17-3. Screw damp assemblies, a. Strap clamp assembly; b. Strap damp assembly-single end, hexagonal nut; c Strap 

clamp assembly- single end, acorn nut; d. Strap damp assembly-single end, hand knob; e. Strap clamp 

assembly-double end, acorn nut; f. Strap clamp assembly— double end, hand knob; g. End hand knob assembly. 

also offer considerable flexibility since individual 
parts are standardized and interchangeable; e.g., 
the hex nut can be replaced by an acorn nut or a 
hand knob, or the single end strap can be replaced 
by a double end strap. The studs are made of high- 
tensile steel, and straps; nuts; spherical washers; and 
clamp rests are made of heat-treated steel with black 

oxide finish. Hand knobs are cadmium plated. 
Single end straps (b, c, d) have machined finger 
grips for easy lateral adjustment. The straps are 
spring loaded (lifted) for quick release and for hold- 
ing them in position when released. Spherical wash- 
ers compensate for irregularities between strap and 
part and ensure rigid holding. Double end straps 



Ch. 17 



? f *fc 




















— c~ 



+ .000 


+ .000 




Courtesy of American Drill Bushing Co. 

17-3 (Cont.j. Screw clamp assemblies, h. Removable damp assembly; i. Swing clamp assembly for reamed hole 

mounting; j. Swing clamp assembly with flange base; k. Hook clamp assembly with socket head cap screw. 

(e, f) are used for clamping flat parts in the position 
shown in the illustrations or can be inverted for 
holding round pieces in V-blocks or in nesting 
arrangements. With open hex nuts (a, b) the as- 

sembly provides maximum clamping range and has 
visual indication of thread engagement. The acorn 
nut (c, e) protects the stud thread from dirt and 
damage, but limits the clamping range. The hand 

Ch. 17 



— c - 

i I 




I i 






FOR # 10 

Courtesy of American Drill Bushing Co. 
Fig. 17-3 (Cont.J. Screw clamp assemblies, i. Hook clamp assembly with stud and hexagonal nut; m. Hinge clamp. 

knob (d, f) provides quick clamping and release with- 
out need of a wrench. With a double end strap, the 
assembly can clamp two parts in one operation, and 
can also be used as a single end strap against a clamp 
rest permanently mounted in the fixture. Single 
end strap -Range: Thread from 1/4-20 to 3/4-10, 
travel G from 1/2 to 1 3/4 inches, capacity / from 
7/8 to 2 inches. Double end strap— Range: Thread 
from 3/8-16 to 3/4 10, travel from 1 to, 1 3/4 inches 
capacity from 1 5/8 to 2 inches. 

End hand- knob clamp assemblies (g) combine the 
maximum clamping range of the hex nut with 
the quick operation offered by the hand knob. 
Rest plate made of heat-treated steel protects fix- 
ture body and assists in locating the strap prior to. 
■clamping, Range: Thread from 1/4-20 to 5/8-11, 
travel from 1/2 to 1 1/2 inches, capacity from 5/8 to 
1 3/4 inches. 

Removable clamp assemblies (h) permit complete 
and fast removal from the fixture of the entire 
assembly. The hardened bottom insert is pressed 
into a recessed hole in the fixture base, and the 
bayonet- type T-bolt is inserted into the slot and 
turned one quarter turn The hand knob, locked 
into place with a jam nut, is used for inserting and 
removing the assembly. The speed handle is for 
clamping and releasing, and can be replaced by a 
flange nut for use where space is limited. Range: 
Thread from 5/8-11 to 3/4-10, capacity from 3 1/8 
to 5 1/8 inches. 

Swing clamp assemblies (i, j) are made both for 
reamed hole mounting and for mounting with a 
flanged base. While all strap clamps can be swung 
free of the part, they require considerable space but 
the swing clamp assembly swings free with much 

less space requirements. The clamping screw has a 
swivel head to protect the surface of the work and 
adjust to irregularities. The arm can be adapted to a 
left or right swing by installing a stop pin in a dowel 
hole to one or the other side of the tail lug. Range: 
Thread on clamping screw from 5/16-18 to 5/8-1 1, 
travel from 5/16 to 11/16 inch, capacity is not 
limited by assembly but is determined in the design 
of the fixture. 

Hookclamp assemblies (k,l), available with socket- 
head cap screw and stud and hex nut, are ideal for 
use where space is extremely limited. The hook is 
spring loaded for quick release and easily swings 
away from the workpiece. The body is an alloy 
steel investment casting with precision ground dia- 
meter designed for reamed hole mounting Range: 
Thread on cap screw from 5/16-18 to 1/2-13, on 
stud from 5/16-18 to 5/8-11, capacity from7/8 to 
1 11/16 inches. 

Hinge clamp assemblies (m) allow fast, and com- 
pletely free, access to work area. They can clamp 
directly on the part or be used for locking a swing- 
ing leaf. A hinged mounted pad bolts to the fix- 
ture. A hand knob swivel screw provides quick 
clamping and release, protects work surface and 
compensates for minor surface irregularities. Range: 
Thread from 5/16-1 8 to 3/8-16, capacity //max from 
1 1/4 to 2 5/8 inches, throat A from 2 to 3 1/2 

Cam Clamp Assemblies (See Fig. 17-4 a through e.) 

Long travel cam clamps (a) have a hand knob and 
a bayonet-type guide groove in the stem. The hand 
knob is pushed straight against the work to close, 



Ch. 17 

i — c — J 

— rB 








Fig. 17-4. *Cam clamp assemblies, a. tLong travel cam lock clamp; b. tCenter cam clamp assembly -single end; c. Center 
cam clamp assembly -double end; d. 'End cam clamp assembly; e. Automatic cam clamp assembly for quick 
release and retraction. 

* Illustrations courtesy of the following companies: T American Drill Bushing Co.; ** Monroe Engineering Products Inc. 

and rotated to clamp and lock. This type of clamp 
combines long travel and quick clamping better than 
any other manual clamping device. The rotating pad 
on the end of the stem protects surface of work- 
piece. Range; Diameter of stem from 3/8 to 1 inch, 
rapid travel from 1/2 to 2 1/2 inches, locking travel, 
equivalent to a cam rise, from 1/16 to 3/16 inch. 

Cam- actuated strap clamp assemblies (b, c, d) are 
available with single end and double end straps and 
center cam, and as single end straps with plain and 
automatic end cam. The clamp strap is actuated by 
a quick-acting clamping and release cam. The strap 
is lifted and carried by a spring. Spherical washers 
permit adjustment of strap to surface irregularities, 

Ch. 17 



Nuts on stud in end cam assemblies allow for adjust- 
ment in height and compensation for wear. Center 
cam, single end -Range: Thread from 1/4-20 to 
5/8-1 i, travel G from i/2 to 1 1/2 inches, capacity 
J from 5/8 to 1 5/8 inches. Center cam, double 
end-Range: Thread from 1/2-13 to 5/8-11, travel 
from 1 to 11/4 inches, capacity from 13/8 to 
1 5/8 inches. End cam-Range: Thread from 1/4-20 
to 5/8-11, travel from 1/2 to 1 3/8 inches, capacity 
from 7/16 to 1 1/16 inches. 

Automatic end cam clamp assemblies (e) have 
single end straps. Cam action automatically re- 
tracts strap after release and brings strap forward be- 
fore clamping in one movement of handle. Handle 
may be mounted on either side of base block. 

Range: Thread from 1/4-20 to 5/8-11, travel from 
1/2 to 1 inch, capacity relative to base from 1 3/8 to 
2 5/8 inches; capacity can be changed by changing 
fixture dimensions. 

Individual Clamping Components (See Fig. 17-5 a 
through d, and Fig. 17-6.) 

All parts incorporated in the assemblies previously 
described are individually available for use on jigs 
and fixtures. Various other parts for similar pur- 
poses include plain and serrated end steel clamp 
straps (a), matching steel and aluminum step blocks 
(b), and chuck jaws (c). 



Fig. 17-S. Individual clamping components, a. f f Plain and step clamp straps; b. **Aluminum and steel step blocks; 
c. "Chuck jaws; d, t Chuck jaw boring positioner. 

'illustrations courtesy of the following companies: 'American Drill Bushing Co.; ''Northwestern Tools, Inc.: ** Monroe 
Engineering Products Inc. 



Ch. 17 

serts are then machined to the desired diameter and 
profile. Range: Clamping diameter from 1 1/2 
to 6 inches. 

Use in pairs to 
support plain 
clamp strap. 

Matched ser- 
rations on all 
step blocks 
permit use of 

mixed sizes. 

Courtesy of American Drill Bushing Co. 
Fig. 17-6. Clamping setup with step blocks and plain and 
step-clamp straps. 

Serrations on blocks and clamps are precision ma- 
chined for perfect fit between half blocks put to- 
gether and between straps and half blocks. They 
are used for setups in machining operations, as shown 
in Fig. 17-6, for temporary fixtures, and as compon- 
ents on universal jigs. Plain and serrated end steel 
straps- Range: Bolt size from 5/16 to 1 inch, length 
from 2 1/2 to 10 inches, width from 1 to 2 inches, 
thickness from 1/2 to 1 1/2 inches. Step blocks- 
Range: Width from 1 to 2 inches, capacity (height 
of clamped part) from 3/4 to 9 inches. 

Chuck jaw inserts (c), to be mounted on the master 
jaws of standard 3-jaw lathe chucks, arc available in 
low carbon steel and 2024- T4 aluminum; in each 
case they are made of bar stock, not cast material 
Since they are soft, they can be machined to fit a 
specific part, thereby converting the chuck into a 
turning fixture. The materials used permit easy 
machining. Steel inserts are preferred for most 
normal applications; they stand up well for medium- 
size production lots; for large production lots, they 
can be carburized and case hardened. Aluminum 
inserts offer some special advantages: They protect 
highly finished machined surfaces and parts made 
from soft materials, and their light weight reduces 
the moment of inertia of the chuck assembly and the 
load and wear on the spindle bearings. Range: 
Length from 2 5/8 to 6 inches, width from 1 to 
2 1/2 inches. 

To assist in the machining of soft inserts, boring 
positioners (d) are available. The boring positioner 
is a flat disc in the form of a three-lobe cam. It is 
placed inside the master jaws of the chuck, and is 
rotated with a screwdriver until the desired diameter 
is obtained. With this position of the boring posi- 
tioner, the jaws are drawn tightly against the edge 
of the positioner to eliminate backlash, and the in- 

Fixed Locating Components 

through k.) 

(See Fig. 17-7 a 

Rest buttons (a through f) are installed in a fixture 
to provide level support of the workpiece and also to 
prevent it from resting on accumulated chips and 
dirt. Rest buttons arc made from heat-treated steel, 
ground to size on principal dimensions. 

Round rest buttons (a, b, c) can be obtained with 
flat or spherical tops and for various methods of in- 
stallation. The most accurate and least expensive 
way of mounting is by a press fit (a, b). Buttons 
with flat head are made with head thickness (dimen- 
sion B) precision ground to final height or manufac- 
tured to 0.010 to 0.014 inch oversize to allow for 
finish grinding after installation in the fixture. A 
less accurate mounting method is by means of a 
screw thread on the shank of the button (c) and 
a tapped hole in the fixture base. These buttons 
are hexagonal and are made with oversize height for 
finish grinding. 

Hollow rest buttons (d) can be mounted hy means 
of a separate flat-head or socket-head screw. The 
button is counterbored for the screw head and pre- 
cision ground to final height. This type of button 
is available in heights up to 2 1/4 inches and is, to- 
gether with the hexagonal screw-mount ed button, 
also used as feet for drill jigs. It is best not to use 
this button with the counterbore in the top post 
tion as small chips and dirt can lodge in the counter- 
bore possibly resulting in inaccuracy when the work- 
piece rests against the button. 

Rest buttons are also made with slip-fit shanks (e, 
f) for quick change. They are secured either by a 
separate lock screw (e) or by a screw thread (f) on 
the outer end of the shank. Slip-fit rest buttons 
require retainer bushings (g, h, i), press- fit mounted 
in the fixture base. Retainers are straight bushings 
(g) or shoulder bushings (h) for use with buttons 
with separate lock screws. The threaded slip- fit 
buttons require retainers (0 with a matching screw 
thread below the bore. Buttons of various types- 
Range: Diameter from 5/16 to 1 5/8 inches, height 
from 1/8 to 1 inch. 

Rest pads (j) are used in lieu of buttons for level 
support of large workpieces in heavy-duty applica- 
tions. They are precision ground, ready for installa- 
tion by means of countersunk, socket-head cap 
screws. Range: LengUi from 2 3/8 to 3 3/8 inches, 
width from 1 to 2 inches. 

Ch. 17 













1/8—1 * 


— B — 
+ .014 





+ .010 
+ .014 

Courtesy of American Drill Bushing Co. 

Fig. 17-7. Fixed locating components, a. Rest button (locator) for press-fit installation; b. Spherical button for press-fit 

installation; c. Threaded hexagonal rest button; d. Hollow rest button, also used as buttons for jig feet; e. Rest 

button for slip-fit installation with lock screw recess; f. Threaded hexagonal rest button with straight shank for 

slip-fit installation. 

Jig legs (k) are available in double end design. 
They are press- fit mounted in the jig base and se- 
cured by screw thread and nut. The small end is 
extended beyond the thread and formed as a small 
jig foot. Range: Thread from 1/2-13 to 3/4-10, 
length of large foot from 2 to 6 inches, length of 
small foot from 1/4 to 5/16 inch. 

Intermediate (Adjustable) Supports (See Fig. 17-8 

a through d.) 

Jack locks (a, b) are used for actuating (lifting and 
lowering) plunger-type intermediate supports and 
locking them in place when lifted to contact with 

workpiece. Typical applications are to support cast- 
ings and other rough workpieces at points between 
the fixed locating points, to eliminate deflection 
during machining. When released, the jack lock can 
be moved freely in either direction. A quick twist of 
the hand knob locks the jack by the expanding 
action of two hardened steel shoes. A lock stop, 
mounted on the side of the jig by one screw, prevents 
the unit from being rotated or pulled out of the 
fixture. Range: Diameter from 0.624 to 1.249 
inches, travel from 3/4 to 1 1/2 inches. 

Spring jack locks (c) are completely self-contained 
units comprising plunger with cap, actuating spring, 
screw with hand knob for release and locking, hous- 
ing and (optional) base for mounting on fixture base. 



Ch. 17 

t- c — - 


- A 




1/16— — 

-.343 RADIUS 

Courtesy of American Drill Bushing Co. 
Fig. 17-7 (Cont.). Fixed locating components, g. Headless retainer for slip-fit rest button; h. Shoulder retainer; i. Threaded 
retainer; j. Rest pad; k. Double-end jig leg. 

They compensate for irregularities in workpiece 
dimensions by automatically adjusting the plunger 
with cap to the work surface by the action of the 
spring. After contact is established, the plunger is 
locked tight by a twist of the hand knob. They are 
designed for fixed spring pressure and for screw- 
adjusted spring pressure. The working mechanism is 
protected against chips and dirt by a long skirt on 
the cap. Range: Travel 5/16 inch, height extended 
from 2 3/8 to 2 9/16 inches. 

Eccentric leveling lugs (d) are circular discs with a 
pivot hole located off -center. They provide positive 
support with precise adjustment. Position after 
final adjustment is permanently secured with a dowel 
pin and two dowel holes are provided for this pur- 
pose. Discs are supplied in soft steel or carburized 
and heat treated. Range: Diameter 1 inch, thick- 
ness (height) from 1/4 to 3/8 inch. 

Locating Pins (See Fig. 17-9 a through o.) 

Locating pins are for the precise locating of parts 
that are already provided with mating holes. Lo- 
cating pins are either pilot pins (a) or full-length 
pins; a pilot pin and a full-length pin are used to- 
gether; the full-length pin "catches" the part first, 
then provides guidance and support as the part is 
lowered to "catch" on the pilot pia Most pins are 
chamfered on the end (b) or bullet nosed <e) to 
facilitate catching and entering. Locating pins arc 
either fully round or relieved by four flats forming 
a "diamond pattern" (b, c). The diamond pattern 
provides the combination of a close tolerance in one 
direction (the major axis in the diamond) with a 
wide tolerance in the other direction (the minor 
axis). Pin diameters (full round and diamond) are 

Ch. 17 













+ .000 

r B+.001 




Courtesy of American Drill Bushing Co, 
Fig. 17-8. Intermediate (adjustable) supports, a. Jack lock; b. Adjustable support with a jack lock installed in a fixture; 
c. Adjustable spring jack locks (press fit and flange base); d. Eccentric leveling lug. 

offered in an "A " and a "B" range; the "B" range is 
0.001 inch smaller than the "A" range. 

Locating pins are press fit (a, b, c, g) or slip-fit 
(e, f) mounted in the fixture. A slip fit is used 
where the pin must be interchangeable and requires 
a matching retainer bushing press-fit mounted in 
the fixture. Slip- fit pins are secured by means of a 
lock screw (e) or by means of a threaded shank and 

a nut (f). Round and diamond pins are available 
with a knurled portion (larger than the pin) for 
embedment in plastic or castable tooling (h, i). 
Locating pins are also used for locating of two fix- 
ture parts relative to each other, which requires mat- 
ing liners <d) in the second fixture part. Standard 
drill bushings are suitable for mating bushings in 
most cases. Range: Most types of locating pins are 



Ch. 17 




h L + .Q0OO 




1 -.0002 

in R PIL0T 



— j f— 1/3 OF A OR I 


60° CENTER- 

1/4 OF A— ■> f— SPHERICAL G-H 

-H r— 1/3 OF A OR B PILOT 


r+ 0000 


— 1/B 1 



-.0003° J e j 1 U F 







"— _* t.0005 
{^ -.0000 

| \ 





g : 

— A 

r +.0000 



^-ed~~M , 





Courtesy of American Drill Bushing Co. 
Fig. 1 7-9. Locating pins. a. Pilot locating pin for press-fit installation; b. Round and relieved (diamond type) locating pins; 
c. (Left) Round and (Right) relieved (diamond type) bullet -nose pins; d. Liner for bullet-nose pin; e. Round and 
relieved (diamond type) locating pins for slip-fit installation with lock screw recess; f. Threaded round and 
relieved (diamond type) locating pins with straight shank for slip-fit installation; g. Stepped round and relieved 
(diamond type) locating pins for press-fit installation; h. Knurled round locating pin. 

available in diameters from 1/8 or 1/4 inch as the 
lower limit, to 7/8 or 1 inch as the upper limit, and 
in lengths from 1/8 to 1 inch. 

Floating pin locators (j), mounted in a press- fit 
bushing, serve the same purpose as the diamond pin, 
providing a close tolerance in one direction and a 

Ch. 17 



♦ 0000 

- 000? 


RO[[ 'IN 

3/16 X 3/1 










-J J 

c » 

2 t 







-, 1 1 , 




- A 

3 CE 

1/8 DIA 1/8 DIA 






Fig. 17-9 {Cont). Locating pins 



Courtesy of American Or HI Bushing Co. 
Knurled relieved (diamond type) locating pin; j. Floating pin locator, comprising pin 
with bushing; k. Slotted hole locator bushing, for use with "L" or "T" pins; 1, Knurled slotted hole 
locator bushing for plastic tooling; m. L-pin; n. T-pin; o. Double-action captive L-pin, 

wide tolerance (1/8 inch total floating travel) in the 
perpendicular direction. Float direction is defined 
and secured by means of a roll pin. Range: Dia- 
meter of floating pin from 1 /4 to 5/8 inch. 

Slotted hole-locator bushings (k), press-fit mounted 
in the fixture base with position secured by means of 
a roll pin, serve the same purpose as the diamond 
pin. With a mating round pin they provide a close 
tolerance in one direction and a wide tolerance 
( 1 /8-inch travel allowed lengthwise in the slot) in the 
perpendicular direction. Slotted hole bushings are 

also available with knurled outer surface (1) for em- 
bedment in plastic orcastable tooling. Range: Slot 
width from 1 /4 to 1 /2 inch for press-fit mounting, 
from 3/16 to 1/2 inch for mounting in plastic. 
L-pins (m) and T-pins (n) are easily removable 
locators for temporary precision alignment of pre- 
drilled workpieces in jigs and fixtures, or for align- 
ment of a drill jig plate on a part after the first 
hole or the first two holes have been drilled. Con- 
ventional L- and T-pins are loose parts and are re- 
moved from the fixture when not in use. They can 



Ch. 17 

be obtained with a cable for permanent connection 
of pin to fixture to prevent loss. Detent pins have 
one or (usually) two spring-loaded balls embedded in 
the pin body to provide a nonpermanent lock and 
protect against accidental withdrawal of the pia' 
Captive locating pins (o) are provided with a bushing 

in which they slide with a precision sliding fit. The 
bushing is permanently installed in the jig with a 
press fit or a slip fit (secured with locking screw). 
Bushings are available with knurled exterior for em- 
bedment in plastic tooling. The pins are captive in 
their bushings. A single-action pin can be pushed 
















Courtesy of American Drill Bushing Co. 
Fig. 17-10. Indexing components.* a. Rotary cam operated tapered (Left), and straight (Right), indexing plungers for 
standard mounting; b. Suggested methods of adapting head of plunger pin to actuating devices; c. Spring-loaded 
straight indexing plunger. 

Ch. 17 



down the full length of the pin, retracted until the 
pilot end of the pin is inside the bushing, and held 
in this position by a groove in the pin. Double- 
action pins have an additional but reversed upper 
groove that limits the downward travel. Range: 
Pin diameter from 3/16 to 1/2 inch, length of travel 
up to 6 inches. L^ and T-pins can be obtained with a 

screw thread instead of the pilot end and used as 
clamping screws, 

Indexing Components (See Fig. 17-10 a through i.) 

Precision made indexing plungers and matching 
bushings are the most critical detail required in the 


-B *■ 



Fig. 17-10 (Cont.). *Indexing components, d. ^Spring plunger; e. tSpring plunger mounted in a blind hole; f. tSpring 
plunger mounted in a through hole; g. § Stainless steel ball plunger; h. tliall plunger detent; i. tSpring 

Illustrations courtesy of the following companies: 'American Drill Bushing Co.; §Morton Machine Works. 



Ch. 17 



1 — . 


*B " 





A ±0002 

6-32x1/4 DEEP 




-49 DIA 

.31 DIA 

500 ±,0002 

Courtesy of American Drill Bushing Co. 
F%. 17-11. Miscellaneous components, a. Toggle clamp of standard design; b. Toggle clamp for heavy duty, built of forged 
parts; c. Vertical angle toggle clamp; d. Push-pull toggle clamp; e. Standard tooling ball; f. Shoulder tooling ball; 
g. Toolmakei's construction ball; h. Tooling ball pad. 

construction of an indexing fixture. Plungers are 
straight or tapered with a 1 5-degree included angle 
(a). Plain indexing plunger units are made without 
actuating devices. The plunger head is soft so that 

it can be machined in accordance with the design of 
the plunger actuator supplied by the customer (b). 
The plunger housing is machined for press-fit mount- 
ing in the fixture. Cam-actuated plunger units are 

Ch. 17 



manually operated. Handle rotation of 180 degrees 
completely retracts plunger. Plunger housings are 
standard mounted in a reamed hole in the fixture, 
and secured in the "whistle notch" by means of a 
nylon tipped set-screw. Housings are also available 
with mounting flange. Range: Housing diameter 
from 3/4 to 2 inches, length up to 3 inches. 

Spring plungers (c through g) lock, support, and 
locate workpicce or fixture parts by spring-actuated 
plungers and require less space than any other de- 
vice. Also used as ejectors, they are available with 
standard end force up to 68 pounds, and with light 
end force up to 3 1 pounds. A nylon plug is inserted 
in the screw thread and provides a positive, vibration- 
proof lock. The normal type is a self-contained unit 
where the spring and plunger are located within a 
threaded" stud; loose parts are also available for 
mounting in a blind or through hole in the fixture 
wall. A modification is the ball plunger (g) where 
the plunger is a steel ball {low alloy steel or stain- 
less steel); for accurate positioning the mating part 
should be provided with a hardened steel detent (h). 
Range: Thread from 6-32 to 1-8 (ball plungers 
down to 4-48), length from 7/16 to 2 13/32 inches. 

Spring stops (i) with circular crowned or rectangu- 
lar tapered pressure heads are used for holding parts 
against locators and for light-duty locking functions. 
Range: Head size from 3/8 to 3/4 inch, end force 
from 1 pounds to 32 pounds. 

Miscellaneous Components (See Fig. 17-11 a through 

Toggle clamps (a through d) are manually operated 
linkage clamps based on the same kinematic princi- 
ple as the eccentric clamp. When clamping, the tog- 
gle link is pushed slightly past dead center and stays 
locked in this position. Locking and release is done 
by a quick swing of the handle. When released, the 
clamping arm is swung at least 90 degrees away fromi 
the clamping position and provides excellent access 
to the clamping area. Normal working position of 
the clamping arm is horizontal (a, b), however, 
models are available with working position of the 
arm 45 degrees up or down (c) or 90 degrees down. 
A push-pull variation of the toggle clamp (d) per- 
forms the clamping by means of a sliding plunger. 
Toggle clamps require relatively large construction 
and operation space; they are manufactured in vari- 
ous series, differing in strength and power. Range: 
Clamping force from 50 to 3000 pounds, capacity 
below horizontal clamp from 11/32 to 3 inches. 

Tooling balls (e, f, g) provide a precision reference 
point relative to a part or a fixture, for the adjust- 

ment of the machine tool spindle prior to critical 
machining operations. The ball and shank are con- 
centric within 0.0002 inch T.I.R. (total indicator 
reading). The standard tooling ball (e) provides 
immediately a "visible" and measurable extension 
of the axis of the bore where the shank is located. 
By an additional measurement taken from the ball 
to the part, the location of the ball center is defined 
and available for axial adjustments. The shoulder 
tooling ball (f) provides a built-in reference dimen- 
sion from the shoulder to the ball center. The tool- 
maker's construction ball (g) provides the same di- 
mension and can be locked in position by a screw in 
the internal thread in the shank. Range: Ball dia- 
meter from I /4 to 1 inch. 

The tooling bail pad (h) is a supplement to the 
standard tooling ball and is used where the part does 
not provide a suitable reference hole for the tooling 
ball shank. The tooling ball can be adjusted to the 
exact position desired and locked securely in place 
by a cap screw lock without marring the tooling 
ball shank. The pad is drilled for mounting on the 
part by a cap screw, and the pad position can be 
secured by dowel pins. 

Cast Iron Fixture Stock Sections (See Fig. 17-12) 

Fixture stock consists of cast iron plates, blocks, 
and profiled shapes and is used in the design and 
building of fixture bodies, thereby saving time and 
expense for patterns. The following sections are 
available: V-, U-, H-, L-, and T-sections, flats, and 
hollow squares and rectangles. T-sections are avail- 
able in equal and unequal shapes. The material is 
high- tensile-strength cast iron; the stock is machined 
square and parallel to within 0.005 inch per foot 
on all external surfaces. The interior of hoEow 
shapes is left unmachined. Range: V-sections have a 
90 degree included angle, an opening width from 
1 1/2 to 4 1/2 inches, and height from 1 1/8 to 3 
inches. Other sections have overall dimensions from 
3 to 8 inches, rectangular hollow blocks have up to 
a 10-inch-width, wall thickness of open shapes from 
5/8 to 1 1/4 inches, wall thickness of hollow blocks 
from 1/2 to 1 inch. 1 

Other commercial products in larger sizes and for 
more general applications are also available. They 
are described in the chapters on fixture bodies, drill 
jigs, universal fixtures, and automatic fixtures. 

Tables with complete dimensions for a number of the basic 
fixture components are found in Eric Oberg and F.D. Jones, 
Machinery's Handbook (New York: Industrial Press, Inc., 
1971) 19thed., pp. 1883-1911. 



\.- :::■'■■■' 

^P I 

Courtesy of EX-CELL-O Corp. 
l*ig. 17-12. Cast lion fixture stock sections, a. U-Sections; b. L-Sections; c. T-Sections (equal); d. T-Sections (unequal); 
e. V-Sections; f. H-Sections; g. Flats; h. Square hollow blocks; i. Rectangular hollow blocks. 




Design Studies I — Drill Jigs 

The Five Basic Design Steps 

From the discussion in Chapter 3, The Fixture 
Design Procedure, it becomes evident that there is a 
great deal of similarity in the design and design 
procedures for jigs and fixtures with respect to the 
principles of locating, clamping, and supporting; the 
principal difference being in the guidance of the 
cutting tool and, to some extent, in the support 
against the cutting forces. As outlined in Chapter 3, 
the systematic design of a fixture is comprised of 
the following five steps: 

1 . Designing a method of locating in the jig or 
fixture which will correctly orient the surfaces on 
the workpiece, for machining or other manufactur- 
ing operations. 

2. Designing a clamping method that will hold 
the workpiece firmly against the locators and 
against the cutting forces. 

3. [f required, designing additional intermediate 
supports that may be needed to prevent the work- 
piece from springing or bending when it is subjected 
to the clamping forces and the cutting forces. 

4. Designing or selecting the cutter guides; for 
drill jigs this means the drill bushings. 

5. Designing the jig or fixture body to consolidate 
ail of the components previously designed, into one 
unified structure. 

These five steps are fundamental and of universal 
validity. In their application, the designer must 
consider the dimensions, material, and weight of the 
part to be handled in the fixture; the already existing 
surface finish and accuracy of its surfaces; the accu- 
racy required in the operations to be performed; the 
quantity to be manufactured; the probability for 
multiple machining; and safety for the operator, the 
equipment, and the part. Furthermore, when work- 
ing on the design, the designer must keep in mind the 

possible procedures that a toolmaker will employ to 
construct the jig or fixture. 

For the purpose of evaluating the degree of sophis- 
tication and perfection to which the fixture should 
be designed, production quantities can be classified 
as follows: 

Type of Production 

Number of Pieces 

Small lot 

up to 40 

medium lot 

from 40 to 100 

large lot 

from 100 to 1000 


over 1 000 

This systematic fixture-design procedure will be 
demonstrated by the application of the five basic 
steps to a number of cases. This chapter covers 
drill jigs; the following chapters will deal with typical 
fixtures for milling and other machining operations. 

Case Series 

Case J, A Jig Plate for a Small Part 

Design a drill jig for drilling two 3/8-inch (10 mm) 
diameter holes in the part shown in Fig. 18-1. The 
part measures 5 by 2 1/2 by 7/8 inches (127 by 64 
by 22 mm), is made of cold-drawn AISI 1030 steel, 
cut to length, finish-machined on the ends, and 
weighs 3 pounds (1 1/2 kg). Holes are to be drilled 
with a twist drill; reaming is not required. The 
quantity to be made is classified as small-lot pro- 
duction and called for the cheapest possible type of 
tooling. Obviously, the size and weight of the part 
presents no handling problem. Surfaces and edges 
are sufficiently flat and straight for locating, sup- 
porting, and clamping purposes. The simplest and 
cheapest type of tooling is a plate jig made to the 
same length and width as the part, as shown in Fig. 
18-2. This solution correlates with the five steps 
as follows: 




Ch. 18 


_1 J 



Fig. 18-1 

Fig. 18-2 

Fig. 18-3 

Fig. 18-1 
Fig. 18-2 
L'ig. 18-3 
Fig. 18-4 

Fig. 18-4 fig- 18-5 

A sample part used for the development of open drill jig designs shown in this group. 

Fig. 18-6 

A jig plate with bushings. 

Clamping a jig plate with stacked parts. 

Sample part, conventionally dimensioned. 
Fig. 18-5. A jig plate with locating pins. 
Fig. 18-6. A modification of the shape of the jig plate. 

1 . The jig is located by laying it on the part and 
lining it up along its edges. The part is further 
located by setting it, directly or indirectly, on the 
drilling machine table. 

2. Clamping is done by auxiliary components, not 
integral with the jig. As shown in Fig. 1 8-3, the jig 
and the part (in this case a stack of two pieces) can 
be clamped on the drilling machine table by a con- 
ventional strap clamp with a bolt and heel block, 
or a simple clamping plate with a stud, strap clamp, 
and heel block could be made. Two low parallels 
under the work will provide clearance for the drill. 
A bolt fastened into the T-slot of the drill press 
table will prevent the work from spinning. 

3. With the substantial thickness of the part and 
its regular shape and flat surfaces, there is no need 
for additional intermediate supports. 

4. For minimum cost, the jig plate is made with 
holes to match the drill to be used, and it is made 
without bushings. This is acceptable for small-lot 

production. For larger quantities, drill bushings 
should be used as shown in Fig. 18-2. The thickness 
of the jig plate can be taken as 3/4 inch (19 mm), 
equaling two times drill diameter, for good bearing 
length (see Chapter 14, the section on Standard 

5. This step is automatically accomplished by mak- 
ing the jig plate in one piece. 

In a conventionally dimensioned part drawing, the 
dimensions a, b, c, and d, shown in Fig. 18-4, will 
be given. For the drawing of the jig plate, the di- 
mensions are transferred to coordinates, as explained 
in detail in Chapter 16. The upper long side and the 
left-hand short side are selected as the axes, so that 
the coordinates represent the distances from two 
edges of the part to the hole centers. Starting from 
this drill jig, which represents the simplest possible 
design, a number of steps can be taken to meet the 
more severe requirements of larger production and/ 

Ch, 18 



or larger dimensions of the workpiece. Since the 
parts have flat, parallel surfaces they can be stacked 
so that more than one piece is drilled at a time, as 
indicated in Fig. 18-3. For other, more advanced 
requirements, bushings are used, as indicated before, 
and the plate is made of a carbon steel in the AISI 
1025 to AISI 1035 range. These steels are inexpen- 
sive and readily machinable and have sufficient nat- 
ural hardness to resist wear and surface damage, 
caused by accidental nicking and scratching, reason- 
ably well. They arc also available cold rolled which, 
for the present purpose, would save the machining 
of the two large surfaces. 

The operation of the jig is improved by the addi- 
tion of locating points acting against two adjacent 
sides of the work, two on the long side and one on 
the short side, as shown in Fig, 18-5, The locat- 
ing points are cylindrical pins, installed with a press 
fit. Such points are cither dowel pins or commercial 
locating buttons (see Chapter 17). Since they act 
against flat, machined surfaces, they are provided 
with flat contact surfaces, as shown. The addition 
of pins makes the locating operation faster, and the 
positive contact with three pins ensures that all parts 
are drilled identically alike. Figure 18-6 shows a 
modified shape of the jig plate, suitable for casting, 
and resulting in a saving in weight for large jigs. 
When it is required to positively clamp the part to 
the jigs, lugs A , shown in Fig. 1 8-7, are added to the 
jig plate to carry finger screws for locking the part 
in position against the locating pins. This eliminates 

Fig. 18-7. A jig plate with clamping means, 

the danger of accidentally losing the correct posi- 
tion. For long and narrow workpieces two lugs on 
the long side are required, to avoid springing (elasti- 
cally deforming) the part. A short and rigid part, 
such as the one considered here, could be sufficiently 
clamped with one lug only, as shown by the dotted 
lines at B. However, sloppy manipulation could 

result in inaccurate clamping. Finally, the jig can be 
provided with feet or legs, as shown in Fig. 18-8, 
which are screwed or pressed in place to the depth 
defined by shoulder A. If screwed, they are locked 
by means of small pins or headless screws. The legs 
are made of a carburi/.ing steel or a tool steel, the 
ends are hardened and, after installation, ground and 
lapped at the ends to exactly the same length. Com- 
mercially available jig feet and legs are described in 
Chapter 17. 

Fig. 18-8. A jig plate with legs. 

Case 2. An Open Drill Jig 

Design a drill jig for drilling two 5/8-inch { 1 6 mm) 
diameter holes in the part shown in Fig. 18-1. The 
part measures 7 by 3 1/2 by 1 5/8 inches (175 by 
90 by 40 mm), is fully machined from a gray iron 
casting, and weighs 10 pounds {5 kg). It is manu- 
factured in repeated lots of 100 pieces each with 
holes to be drilled and reamed. 

The quantities required can be classified as up- 
per limit of medium-lot production, and justifies a 
complete jig with all components integral with or 
attached to the jig, but without devices for auto- 
matic action. With proper design, such tooling will 
ensure interchangeability of the parts independent 
of the operator's skill and care. Application of the 
basic steps leads to the type of jig known as the 
"open drill jig" which is, essentially, a plate jig with 
the part suspended underneath, and standing on legs, 
as shown in Fig. 18-9. Since it is open to one side, 
it is suitable for casting, because it requires little or 
no core work. The design shown is cast, and pro- 
ceeds as follows: 

1 . Holes can be drilled from one side. The part 
has a machined flat surface that is perfect for locat- 
ing against a machined surface on the jig plate, and 
regular edges for side location with two plus one 



Ch. 18 

— i 

IU- i j 1 — » 

! ! i I i 

Fig. 18-9, An open drill jig. 

locating pins. The jig plate with pins can now be 
drawn. Minimum thickness is 1 inch (25 mm), and 
the machined face is raised 1/4 inch (6 mm) to pro- 
vide a clearance all the way around for machining. 

2. The part must be clamped from below against 
the jig plate. It would appear natural to provide a 
strap clamp at each end; however, the part is so 
rigid that it can stand up to the pressure from the 
drill with one strap clamp only, located at the center. 
This solution requires substantial dimensions. The 
strap is 2 3/4 inches (70 mm) wide by I inch (25 
mm) thick and is slotted for the 7/8-inch (22-mm) 
clamping screw so that it can be pulled back and 
clear the locating area for inserting and removing the 
part. The heel block for the strap is provided as a 
downward extension of the jig plate. Three finger 
screws are provided for locking the part against the 
locating pins. 

3. No additional supports are required because the 
part is already well supported over its entire area. 

4. Drill bushings are inserted in the jig plates. The 
bushings are press-fit wearing bushings for use with 
slip bushings for drilling and subsequent reaming. 
The length of each bushing is 1 1/4 inches (two 
times hole diameter). 

5. 1/4-inch (6-mm) bosses are added on the upper 
side of the jig plate to avoid protruding bushings. 
Vertical side walls are formed along the perimeter of 
the jig plate, to carry the finger screws. On the four 

corners, the side walls are formed into legs with 
angular (L-shaped) cross section, and made long 
enough to lift all components clear of the machine 
table. Legs are tapered 15 degrees for maximum 
rigidity with minimum weight, as shown in the 
sketch in the upper right-hand corner. Width b at 
the lower end is 1 1/2 times thickness a, which again 
depends on the jig size, taken as overall face area 
(length times width of the jig plate), as follows: 

Face Area of Jig 

Thickness a 

Square inches 




up to 6 
6 to 60 
over 60 


4000 to 40,000 

over 40,000 

5/16 to 3/8 
1/2 to 5/8 
3/4 to 1 1/2 

8 to 10 
13 to 16 
19 to 38 

This jig measures 10 1/2 by 7 1/2 inches (270 X 190 
mm), and the legs are 3/4 inch (19 mm) thick. Pads 
A are provided on one long and one short side, to 
support the casting when laid out and machined. A 
4-inch (lQO-mm)-long handle is cast on one end to 
give the operator a secure grip during drilling and 

Case 3. An Open Jig with an Intermediate Support 

Design a jig for a bracket with a large boss. The 
part is shown in chain-dotted lines in Fig. 18-10. 
It is a gray iron casting, approximately 13 inches 

Ch. 18 



Fig. 18-10. Plan and elevation of an open jig with an intermediate support. 

(330 mm) long and 8 inches (200 mm) wide, weigh- 
ing 35 pounds (16 kg), with a large boss for a 2 1/4- 
inch (57~mm)-diameter bearing hole A, to be faced 
on both ends. It has a flange, already machined flat 
on its free side, to be provided with three screw holes 
B and two dowel-pin holes C, bosses for screw holes 
B to be spot faced. The initial lot will be 300 pieces 
and may be repeated at a later date. The awkward 
shape, weight, and quantity, call for a good jig with- 
out excessive frills. All operations, except a few 
facings, can be done from one side which also offers 
an excellent, flat, machined locating surface. The so- 
lution is an open jig that can be turned upside down 
for the few facing operations from the other side. 

1 . The jig plate can now be drawn. It is offset 
(Z-shaped) to conform with the height difference 
between the flange and the end of the boss and is 
provided with a raised, machined locating surface to 
receive the machined surface on the flange. Endwise 
locating is against two locating pins at the small end 

of the flange, with a screw at the other end to en- 
sure contact. The pins are round (no flats) because 
Hil'v hear on the unmachined edge of the flange. 
Crosswise, the boss is located between two screws 
which allows for centering the boss with respect to 
the bushing for hole A, and thereby adjusting for 
possible variations in the castings. 

2. The part is clamped against the jig plate with 
three strap clamps D, measuring 1 1/2 by 5/8 inches 
(38 X 16 mm) with slots for 5/8-inch (16-mm) 
clamping screws to allow them to be pulled back 
and clear the part. 

3. The boss is overhanging and not sufficiently 
supported against the heavy cutting-tool pressure to 
which it is exposed. No possibility exists for sup- 
porting it on the free side, but a substantial inter- 
mediate support is provided by the 3/4-inch (1 9-mm) 
pressure screw E, carried by a 1 1/2 by 3/4 inch 
(38 by 19-mm) strap F which is screwed to the jig 
by two 5/8-inch (16-mm) screws G. Screw E sup- 
ports the rib at a point as near as possible to the 
boss. Strap F has a hole at one end and a slot at the 



Ch. 18 

other end, so that it can be swung clear of the part 
by loosening, but not removing, the screws G. 

4, Holes B and C are plain drilled holes and re- 
quire press-fit wearing bushings. Hole A is first 
drilled 1/8 inch (3 mm) undersize, resulting in 1/16- 
inch (1.5-mm) stock allowance for the final opera- 
tion, which is done with a chucking or machine 
reamer. These operations require one press-fit liner 
bushing in the jig and two slip bushings for, respect- 
ively, the drill and the chucking or machine reamer. 
The tools for the facing of A , and the spot facing of 
B, are guided by pilots in the already finished holes, 
and do not require bushings. 

5. A long and a short leg is provided at each cor- 
ner. The overall dimensions of the jig are 17 1/2 by 
10 1/2 inches (450 by 270 mm) which calls for 3/4- 
inch (19-mm)-thick metal in the legs, The jig plate 
is 7/8-inch (22-mm)-thick. Compared to the 1-inch 
(25-mm) thickness used in Case 2, this may appear 
to be on the low side, but the plate is substantially 
strengthened, first by its Z-shape which virtually 
makes it a structural beam, and also by the boss for 
the big bushing. Lugs, bosses, and pads are provided 
for bushings, screws, and clamps. Windows are cored 
out in the large flat panels for weight reduction. 
The result is a cast fixture that is strong, rigid, and as 
light as it can be under the circumstances. 

Case 4. A Modified Procedure for Case 3 

A drastic saving in tool cost, bought by an in- 
crease in the total machining time, is accomplished 

by a combination of turning and drilling operations. 
After a preliminary layout for the center, the large 
boss is drilled, perhaps bored, and then reamed and 
faced, either on a faceplate in a large lathe, or pref- 
erably, on the horizontal table of a vertical boring 
mill (VBM) or a vertical turret lathe (VTL). Holes 
B and C are drilled by using the jig plate shown in 
Fig. 18-11. The jig is located by a plug in hole A, 
and manually aligned with the periphery of the 
flange; the assembly is then clamped on the drill 
table by a strong strap with a jack screw for support- 
ing the overhanging end . 

Case 5, Design of a Closed Jig (Box Jig), of a Rela- 
tively Simple Type, with Positive Locating Means 

Design a drill jig for drilling the four 5/8-inch 
(16-mm) holes in the part shown in Fig. 18-12. 

The part measures 6 by 3 1/2 by 1 3/4 inches 
(150 by 90 by 45 mm). It is fully machined from 
a gray iron casting and weighs 10 pounds (4.5 kg). 
The quantity is small -lot production and calls for an 
inexpensive jig. All holes are drilled; their relative 
positions must be accurately maintained, but their 
location relative to the outline of the part is not 
critical. Holes A are drilled through, but holes B 
and C are blind holes and must be drilled from 
opposite sides. 

When holes must be drilled in several directions, 
and bushings are required, the drill jig must be of 
the closed, or box, type, because it has to be turned 
around for the drilling operations. A closed, or box, 


Fig. 18-11. A jig plate with means for locating from a bored hole. 

Ch. 18 






Fig. 18-12. A sample part used for the development of the 
closed drill-jig design shown in Figs. 18-13 
thiough 18-19. 

jig can be described as an open jig with a floor. 
Since the jig, more or less, completely embraces the 
part, a door, gate, or port must be provided to get 
the part in and out. With this difference in mind, 
most design features in closed jigs are the same as in 
open jigs. 

The simplest solution is to make two jig plates 
as in Case 1 , one for each side of the part, and 
build them together to form a closed jig, as shown 
in Fig. 18-13. The part is placed between the two 
jig plates and is located by two plus one locating 
pins, which have flats to engage with the machined 
edges of the part. The jig plates are held in their 
proper position by large-diameter dowel pins which 
have a press fit in the lower plate and a sliding fit in 
the upper plate. To prevent separation, a screw 
with a large flat head or a fully countersunk, hex- 
agonal socket-head screw is installed in one of the 
dowel pins. Holes are drilled in the lower plate 
opposite the bushings for holes A to provide clearance 
for the drill and for the escape of chips. 

Case 6. A Closed Jig with all Components Integral 
with or Attached to the Jig 

Design a drill jig for the part specified in Case 5 to 
be manufactured in repeated lots of 100 pieces each. 
The design proceeds according to five basic steps and 
results in the jig shown in Figs. 18-14 and 18-15. 

1. The part is located with its flat side on the 
machined pad R of the lower plate /,, and sideways 
and endwise against two plus one locating pins with 
flats to match the machined side and end surfaces of 
the part, and is locked against the pins by two 
1/2-inch (13 -mm) screws U and Q. Obviously, one 
of these screws may block the part from entering, 
once the jig is fully designed and its closed character 
has become apparent. However, this problem will 
be solved in a later step. 

— I ■=-:" 

Fig. 18-13. A simple closed jig with locating pins. 

2, The part is clamped against the pad R, by 
two 5/8-inch (16-mm) screws J, from above. The 
illustration shows an additional screw / (in dotted 
lines) to indicate that for parts of small dimensions 
one clamping screw in a central position may be 


.4 ..J- i. 11 


ft 17 ! •?. n r, \ ft (■ r^* 

1 j * 1 «?i 5 I 5 1 1 1 f 

■ j_;i ■ " 

Fig. 18-14. A well-dcvclopcd closed jig. 

3. The part is so effectively supported that no 
inU-tnu'tliiLlc supports are needed. 

4. 5/8-inch (16-mm) press-fit wearing bushings are 
provided, three from above and one from below. 
Bushings are 1 1/4 inches (32 mm) long for a bear- 
ing length of 2 times the drill diameter. 

5. Upper plate K can now be drawn; thickness is 
1 1/4 inches (32 mm), equal to bushing length. No 
end clearance is required for burrs and chips, be- 
cause there is a space between the pan and the plate. 






1 \ 




Fig. 18- IS. A closed jig supported on parallels. 



Ch. 18 

Side plates, also marked L (left side of the illustra- 
tion), are made integral with lower plate /, ; the 
complete lower part can be machined from a solid 
steel block or from a U-shaped gray iron casting. 
The locating pad R has a clearance groove on each 
side. Wall thickness is a uniform 1 1/4 inches (32 
mm). The upper plate is fastened with 1/2-inch 
(12 mm) screws M, and secured in position with 
dowel pins N. To carry the locking screw Q, a 
swinging arm P is provided, which can be swung out 
of the way for loading and unloading the jig. For 
the drilling of hole C in the underside of the part, 
the jig is turned upside down and placed on parallels 
D (Fig, 18-15). The addition of the machining strips 
/ will save the finish machining of some quite large 
areas of the jig. 

Case 7. A Closed Jig with Legs 

Design a drill jig for the part specified in Case 5, 
with integral legs for the two operating positions. 

The jig is shown in Fig. 18-16. Steps 1, 2, 3, and 
4 are the same as in Case 6. In Step 5, a boss for 
the hushing and four foot pads are added to the 
lower part; four legs and a machining pad around 
the bushings and the clamping screws are added to 
the upper plate. Machined surfaces are marked f. 



Fig. 18-16. A closed jig provided with feet and legs. 

The screw Q is now carried by a 1 1/4- by 5/8-inch 
(32- by 16-mm) swinging strap E, which is supported 
at both ends and, therefore, provides a more rigid 
and secure position for the screw. A handle 5 is 
added on the end of the jig to give the operator a 
safe grip. 

Case 8. A Closed Jig Designed as an Improved Type 
of Leaf Jig 

Design a drill jig for the part specified in Case 5, 
with a swinging leaf, but without clamping means in 

the leaf. While it may be a convenience to use a com- 
mercial leaf jig with the clamping screw, or screws, 
placed in the leaf, it also has its valid objections. 
The clamping pressure is carried by the leaf and 
causes an elastic deflection; if the tight fit in the 
hinge and in the seats against the jig body is loos- 
ened by use and wear, thejposition of the leaf may 
shift when the clamping screw is tightened, causing 
inaccuracy in the direction and location of the drill 
bushings and thus faulty work. For best results it is 
therefore generally recommended to separate bush- 
ings and clamping means and to use the leaf for only 
one of these two types of components, as shown in 
Fig. 18-17 where the clamping is done by two 1 1/2- 
by 7/8-inch (38- by 22-mm) strap clamps G with 
1/2-inch ( 12-mm) screws, one at each end. As usual, 
the straps are slotted for easy withdrawal. This ar- 
rangement is always recommended when the clamps 
have to take a heavy drilling load as, for example, in 
operations using a multiple-spindle drilling machine. 






i ii it ft s n ! it 

Fig. 18-17. A closed jig with a leaf and clamps. 

The leaf is here a 1 1/2- by 1-inch (38- by 25-mm) 
strap, carrying the bushing for hole B and locked by 
means of a thumb screw H, sometimes formed as a 
quarter-turn screw. 

Case 9. A Closed Jig with a One-piece Body and 
No Swinging Parts 

Occasionally, it is possible to design a jig that is 
sufficiently closed to carry bushings for all required 
drilling directions and which still provides an open- 
ing, a port, large enough to bring the part in and 
out. Essentially this depends on the hole location in 
the part. Chances are best if the holes are placed 
near the edges. 

An example is shown in Fig. 18-18. The part is 
the one specified in Case 5. An examination of 

Ch. 18 



y f±^\ 

Ftg. 18-18. A closed jig with a port for loading and 
unloading the part. 

Fig. 18-12 shows that hole B is located fairly close 
to the two edges. The jig is now designed with only 
hole B drilled with the jig upside down, and the 
other holes with the jig in the upright position. The 
bushing for hole B is carried by a boss held on a 
bracket D strengthened by a rib B. With the clamp 
to the right withdrawn, the end of the part can be 
lifted and the part pulled out in a tilted position. 
There must be enough clearance, not only for the 
part, but also for the operator's fingers. The de- 
signer is warned against overoptimism in this re- 
spect. In reality, parts are always larger and clear- 
ances smaller, than they appear to be in a drawing. 

Case 10. A Closed Jig for Drilling from Four Sides 

Design a drill jig for the part specified in Case 5 
with an additional hole in the long side and one in 
the end. 

The jig with the part is shown in Fig. 18-19. Steps 
1 and 3 are the same as in Case 8. In Step 2, clamp 

Fig. 18-19. A closed jig for drilling from four sides. 

H is moved over 7/8 inch (22 mm) to clear the new 
bushing for the end hole, and clamp G is moved 
opposite to maintain the balance of the clamping 
forces. In Step 4, bushings E and F are added for 
the side and end holes. In Step 5, additional pads 
are added to act as feet for the drilling operations 
through bushings E and F. 

General Definitions and Classifications 

Drill jigs are used exclusively for drilling, reaming. 
tapping, and facing operations. Whenever a com- 
bination of these operations is required on a part, it 
is usually possible to design one single drill jig for 
all of them. Drill jigs may be classified as open 
jigs and closed jigs. Some subclassifications within 
these two main classifications can also be made. 
The open jig has all bushings mounted in the same 
plane and with parallel axes. It has no removable 
walls or leaves, thus it is easy to insert and remove 
the part. The simplest type of open jig is the tem- 
plate jig, widely used in the aircraft industry, con- 
sisting of a large sheet of aluminum, magnesium, 
fiber, or laminated plastic with the necessary bush- 
ings, and the means for locating it on the part and 
holding it in position. 

The most typical form of open jig in the average 
machine shop is the plate jig; it is applied to, and 
supported by, the work. It may require clamping, 
but in many cases this type of jig is used on a 
finished portion of the part which provides means 
for holding the jig in position. The next step in 
development is the plate jig with feet, with the part 
clamped below the jig plate. If the four feet are 
replaced by two parallel walls, the jig becomes a 
channel jig. If two more side walls are added, the 
jig may still be a plate jig, but if these side plates 
now are used for installation of drill bushings, then 
the jig has developed into a box jig. 

The open channel jig, as well as any one of the 
more closed type jigs, can be provided with a cover, 
often in the form of a leaf, for fast operation. The 
box jig is frequently formed as a tumbling jig, that is, 
it has feet in several directions and is simply turned 
over 90 degrees or 1 80 degrees when the next side 
is going to be drilled. If there are hole axes at angles 
other than 90 degrees, the jig can be supported in a 
cradle, and if there are a large number of axis direc- 
tions, the jig will be made as an indexing jig. 

Operating with Drill Jigs 

Before a jig is designed, it must be decided whether 
the drilling operation will go to a fixed-spindle drill 
press or to a radial drill. Small parts that are easy to 



Ch. 18 

handle should always be routed to a fixed-spindle 
drill press where the jig with the part is moved so as 
to bring one bushing at a time in line with the drill 

The moving of the part is usually done manually, 
but when necessary, is assisted by a hoist or a crane. 
The use of an air cushion table greatly facilitates the 
shifting of heavy tooling and is gradually gaining 
acceptance in workshops. If there are holes in more 
than one direction, the jig is designed either as a 
tumble jig or as a bracket-type indexing jig, depend- 
ing on the clamping possibilities found in the part. 

Large and heavy parts go to the radial drill because 
this machine has a spindle that is easily moved from 
one hole position to the next, which permits the 
jig to be clamped to one of the machine tables. If 
the part has holes in more than one direction, there 
is again the choice between a tumbling jig and an 
indexing jig. A less than clearcut situation arises 
with medium-size parts, parts that can be manually 
handled on the table of an ordinary drill press, but 
not without some physical effort by the operator. 
The tendency is to prefer the radial drill if it is 
available. Modern radial drills have ample rigidity 

and a larger range of speeds and feeds and usually 
more poweT than drill presses of similar spindle 
dimensions, also all handles and levers for the control 
of the machine are located within easy reach of the 
operator. They are designed for convenient and fast 
manipulation with a minimum of physical effort, 

A most important safety rule applies to the man- 
ual handling of drill jigs. The torque exerted by 
even a relatively small drill exceeds what can be 
safely held by hand. Any drill jig, If it is not 
physically clamped to the machine table, must be 
positively restrained against rotation. A stop block 
may be sufficient for this, but a straight bar or rail, 
contacting one full side of the jig, is better. A set of 
two such parallel rails, with the jig sliding between 
them, is frequently an excellent solution. A rule-of- 
ihumb says that the work can be held by hand when 
drilling holes of 1 /4-inch (6-mm)-diameter or less. 
Even in this case the jig must be of such a shape and 
size that the operator has a good grip on it, or it 
must be provided with a handle. 

The planner must recognize that the drill bushing 
steals some of the available length of drill spindle 
travel. The following values are average and common: 

Fig. 1 8-20. Using a medium-size drill jig. 

Courtesy of Le Blond Inc. 

Ch. 18 



Maximum Drill Diameter^ 

Drill Spindle Travel 










10 x A = 2 1/2 
8 x.4 = 6 
S xA = 10 


Typical examples of medium- and large-size drill- 
jig work are shown in Figs. 18-20 and 18-21. The 
first illustration shows a medium-size part that is to 
be drilled in two directions. In the position shown, 
the jig is supported on a block of such thickness that 
the height to the top of the jig equals the length of 
the jig. When the jig is turned 90 degrees for the 
drilling of the end hole, the height is the same as 
before, and no adjustment of the clamps is needed. 
The second illustration shows a box jig with a 
separate jig plate. The jig is mounted on the index- 
ing table of a large work positioner that is not part 
of the jig. 

Placement of Jig Bushings 

The fixture designer has no option with respect 
to the placement of the bushings, because they are 

determined entirely by the drawing of the part. A 
recurring problem is that holes are so close together 
there is no room for the drill bushings. The various 
possible solutions are shown in Fig. 18-22. The use 
of thin- wall bushings may solve the problem, or two 
standard headless or headed bushings are each ground 
with a flat side and are installed with the two flat 
sides in contact. In extreme cases it is necessary to 
include two or more holes in one single insert. 

The distance from the end of the bushing to the 
surface of the work is important. Various possible 
arrangements are shown in Fig. 18-23. For maxi- 
mum accuracy, the bushing should contact, or almost 
contact, the work to provide maximum support and 
guidance to the drill. When full contact with a 
tight close-up fit on the surface of the part can be 
established, this type of fit is preferable because it 
positively prevents chip jamming below the bushing. 
It can be achieved with the use of the threaded type 
of bushing shown in Fig, 14-1 5d, and later, in Fig. 
18-28, view A, and also in many cases with plate 
jigs. In all other is preferred, even necessary, 
to maintain a short clearance between the bushing 
and the surface. The distance is taken as 0,5 times 
drill diameter for materials that produce short chips, 

Courtesy of Le Blond Inc. 

Fig. 18-21. Drilling a large part with a drill jig plate. 



Ch. 18 




Courtesy of American Drill Bushing Co. and Technological Institute, Copenhagen 
Fig. 18-22. Placement of drill bushings in a limited space. 

and 1 to 1.5 times drill diameter for materials that 
produce long chips. Materials with long chips can 
also be drilled with the bushing in contact with the 
work. In this case, the chip is continuously guided 
up to the surface of the jig. Excessive chip clearance 
is not used because it reduces the guiding effect of 
the bushing. Burr clearance for highly ductile ma- 
terials, such as copper, is 0.5 times drill diameter. 

Drilling on no n -perpendicular surfaces is facilitated 
(see Fig, 18-24) by carrying the bushing down to the 
work surface and cutting the end to the contour of 
the workpiece. In applications of this nature, the 
drill point has a strong tendency to skid or wander. 
For this reason, the distance between the bushing 
and the workpiece must be held to a minimum so 
that the full guiding effect of the bushing can be 

















£ : ig. 18-23. Drill bushing end clearance. 

Courtesy of American Drill Bushing Co, 

Ch. 18 




















Courtesy of American Drill Bushing Co. 
Fig. 18-24. Bushings for drilling through non-perpendicular surfaces. 

obtained. The side load exerted by the drill in 
applications of this type is usually concentrated at a 
point near the drill-exit end of the bushing and 
causes accelerated bushing wear. Except in short 
production runs, the use of fixed renewable bush- 
ings should be considered, to simplify replacement 
of worn bushings and to facilitate proper orientation 
of the bushing with respect to the contoured work 
surface. When press-fit bushings are used, bushing 
end contours should be applied after the bushings 
are installed in the jig plate to assure proper contour 
placement with respect to the workpiece. 

The Open Plate Jig, Reversible Jigs 

This type of jig is simple, inexpensive, efficient, 
and can, if made reversible, be used for the drilling 
of matching parts. The ring-shaped jig shown at A 
in Fig, 18-25 is used for drilling the stud bolt holes 
in a cylinder flange and also for drilling the cylinder 
head, which is bolted to the cylinder. The position 

of the jig when the cylinder flange is being drilled is 
shown at B. An annular projection on the jig fits 
closely in the cylinder counterbore to locate the jig 
concentric with the bore. As the holes in the 
cylinder are to be tapped or threaded for studs, a 
tap drill, which is smaller in diameter than the bolt 
body, is used and the drill is guided by a removable 
bushing of the proper size. Jigs of this type are 
often held in position by inserting an accurately 
fitting plug through the jig and into the first hole 
drilled, which prevents the jig from turning with 
relation to the cylinder, when drilling the otheT 
holes. When the jig is used for drilling the head the 
opposite side is placed next to the work, as shown 
at C. This side has a circular recess or counterbore, 
which fits the projection on the head to properly 
locate the jig. As the holes in the head must be 
slightly larger in diameter than the studs, another 
sized drill and a guide bushing of corresponding size 
are used. The cylinder is, of course, bored and the 
head turned, before the drilling is done. 

A c 

Fig. 18-25. A reversible, open-plate jig. 



Ch. 18 


Fig. 18-26. An open-plate jjg with a centralizes 

The jig shown in Fig, 10-46 is an open, but not 
reversible, plate jig. The jig shown in Fig. 18-26 at 
A and B is a centralizing open-plate jig and is used 
for drilling the ring shown at C. For centralizing 
the jig on the work, it has three plungers ZJ, which 
are held against the conical point of wing screw E 
by springs F, In operation, the wing screw E is 
turned back until the plungers D are well within the 
body G, at points H. The ring C is then slipped on 
and the wing screw is turned down until the plungers 
D are forced out and into contact with the inside 
surface of the ring. The ring is then drilled on a 
sensitive drilling machine, e.g., an upright drilling 
machine with hand feed only. 

The Box Jig 

The leaf jig is a commercially available box jig and 
one example, designed for drilling a hole having two 
diameters through the center of a steel ball, is shown 

in Fig. 18-27. The work which is shown enlarged at 
A, is inserted while the cover is thrown back, as 
indicated by the dotted lines. The cover is then 
closed and tightened by the cam-latch D, and the 
large part of the hole is drilled with the jig in the 
position shown . The jig is then turned over and a 
smaller drill of the correct size is fed through guide 
bushing B on the opposite side. The depth of the 
large hole could be gaged for each ball drilled, by 
feeding the drill spindle down to a certain position, 
as shown by graduation or other marks, but if the 
spindle has an adjustable stop, it is preferably used. 
The work is located in line with the two guide 
bushings by spherical seats formed in the jig body 
and in the upper bushing, as shown. The work can 
be inserted and removed quickly, and a large number 
of balls can be drilled in a comparatively short time. 
A typical, custom designed box jig is illustrated in 
A in Fig. 18-28, where A, B, and C show the three 
positions in which this jig is being used. A is the 





Kig. 18-27. A box jig with leaf. 

Ch. 18 



Fig. 18-28. A typical box jig for drilling in three different positions: A, B, and C. 

loading and unloading position and B and C are the 
two different drilling positions. The work, in this 
case, is a small casting with its form indicated by the 
heavy dot-and-dash lines. This casting is drilled at 
a, b, and c, with the two larger holes a and b finished 
by reaming. For inserting the work, the hinged 
cover of this jig is opened by unscrewing the T- 
shaped clamping screw s one-quarter of a turn, which 
brings the screw head in line with a slot in the 
cover. The casting is clamped by tightening this 
screw, which forces an adjustable screw bushing g, 
down against the work. As this bushing is adjustable, 
it can be set to give the right pressure, and, if the 
height of the castings should vary, the position of 
the clamping bushing is easily changed. 

The .work is properly located by the inner ends of 
the three guide bushings a L , b lt and c x and also by 
locating screws / against which the casting is held by 
knurled thumbscrews m and n. When the holes a 
and b are drilled, the jig is placed with the cover 
side down, as shown in B, and the drill is guided by 
removable bushings, one of which is shown at r. 
When the drilling is completed, the drill bushings 
are replaced by reamer bushings and each hole is 
finished by reaming. The small hole c, is drilled in 
the end of the casting by simply placing the jig on 
end as shown in C. Box jigs which have to be 
placed in more than one position for drilling differ- 
ent holes are usually provided with feet or exten- 
sions, as shown, which are accurately finished to 
align the guide bushings properly with the drill. 

These feet extend beyond any clamping screws, 
bolts, or bushings which may protrude from the 
sides of the jigs, and provide a solid support. When 
inserting work in a jig, care should be taken to re- 
move all chips which mighl have fallen upon those 
surfaces against which the work is clamped and 
which determine its location. 

Another jig of the box type, which is quite similar 
to the one shown in Fig. 18-28, but is arranged 
differently, owing to the shape of the work and 
location of the holes, is shown in Fig. 1 8-29. The 
work has three holes h, in the base, and a hole at i 
which is at an angle of 5 degrees with the base. The 
three holes are drilled with the jig standing on the 
opposite end y , and the angular hole is drilled while 
the jig rests on the four feet k, the ends of which 
are at such an angle with the jig body that the guide 
bushing for hole i is properly aligned with the drill. 
The casting is located in this jig by the inner ends of 
the two guide bushings w and the bushing o, and 
also by two locating screws p and a side locating 
screw q. Adjustable screws f and f, in the cover 
hold the casting down, and it is held laterally by the 
two knurled thumbscrews u and v. 

Jigs for Angular Drilling 

When the work is to have angular holes, that is, 
holes that are to be drilled at an angle with its basic 
surfaces or planes, the jig must be supported in an 
inclined position. For drilling only one angular hole, 



Ch. 18 



tHfA — — * -r '-VU 

Fig. 18-29. A box jig for a small casting. 

or a group of such holes with parallel axes, the jig 
is designed as shown in Fig. 1 8-30. The work, shown 
here as a rectangular block, has one angular hole 













Fig. 18-30. A jig for drilling holes at an angle. 

Fig. 18-31. A jig for drilling holes at an angle, and provided 
with a separate base (a stand). 

which is to be drilled through the bushing A, and 
the feet on the opposite side are machined to a 
plane perpendicular to the axis of the bushing, as 
indicated by the angle a. The feet B are machined 
to a plane parallel to the faces of the work and are 
used for supporting the jig for the drilling of per- 
pendicular holes. When it is required to drill one 
hole, or a group of parallel holes, at an angle and 
other holes perpendicular to the face of the work 
and from the same side, an arrangement such as that 
shown in Fig. 18-31 is needed. The bushing for the 
angular hole is A . On the opposite side, the jig has 
feet of equal length for support when the perpendi- 
cular holes are drilled. A separate base (also known 
as a stand, an angle block, or a cradle) B is provided 
to support the jig in the required inclined position 
for drilling through bushing A. Separate bases are 
used not only for angular drilling, but sometimes 
to accommodate the jig in cases where it would 
be inconvenient to provide the jig with either feet, 
finished bosses, or lugs, for resting directly on the 
drilling machine table. 

The use of a separate base is applicable to jigs of 
almost any size. It does require handling and is 
therefore constructed as light as possible, with a 

A B 

Fig. 18-32. A jig with a swinging leg for drilling at various angles. 


Ch. 18 



large cored hole in the panel and with ribs and webs 
for rigidity. 

Manual handling is greatly reduced by the use of a 
jig with a swinging leg as shown in Fig. 18-32. This 
jig is designed for the drilling of two holes, one of 
which is at an angle. When drilling the straight hole, 
the jig is in the position shown at A ; for drilling the 
angular hole, the operator simply lifts the front of 
the jig, and the swinging leg C falls, bringing the jig 
into the position shown at B, and places the hole to 
be drilled in line with the drill. By using this jig, 
extra parts, such as a separate base or angle block, 
are eliminated, and the jig is very quickly moved 
between operating positions. 

Jigs for Large Work 

When a jig of large dimensions and weight is to be 
turned over, either for the insertion or removal of 
the work, or for drilling holes from opposite sides, it 
is advantageous to have a special device attached to 
the jig for turning it over. Figure 18-33 shows one 
such arrangement for use where a crane or hoist is 


lig. 18-33. A jig for heavy work, provided with trunnions 
for turning. 

available. A represents the jig which is to be turned 
over. The two studs B are pressed or screwed into 
the jig in convenient places, as nearly as possible in 
line with a gravity axis. These studs then rest in 
yoke C, which is lifted by the crane hook placed 
at D. The jig, when lifted off the table, can then 

easily be swung around. The yoke is made of 
round machine steel. 

For work of medium size and weight (in the 
range from 8 to 25 pounds) where crane assistance 
is not feasible, much hard work can be saved and 
production increased by outfitting the jig with rock- 
ers where that can be done without interfering with 
the drilling operation. An example is shown in 
Fig. 1 8-34. The work requires drilling from two 
opposite sides, as indicated by the bushings and 
legs shown, and the third side is available for the 
rockers. They are made from steel plate, machined 
toa radius, and attached with screws. The machining 
of the curved contour does not require high pre- 
cision, since the jig does not rest on the rockers 
during drilling operations. 

The Vise as a Drill Jig 

The machine vises such as are used for milling or 
planing operations may be used for drilling when 
they are provided with attachments for holding drill 
bushings or locating stops. By using suitable plates 
in these jigs, many odd-shaped pieces can be drilled, 
and Fig. 18-35 is a typical example. The method of 
using this plate is shown by the illustration. Bush- 
ings A are placed in plate B at the proper location 
to guide the drills into the work. The plate is 
screwed on top of the vise, stop C is adjusted to the 
proper location, and the work D is placed in the 
vise against the stop, after which the holes are 
drilled . 

Another example of drilling in a vise is shown in 
Fig. 18-36, where a number of holes are drilled 
around a circle. The work is gripped between the 
jaws in the vise proper, and a bushing plate is located 
by pins A and B in the vise. By sliding the vise to 
various positions the holes are drilled in the usual 
manner. This bushing plate is removable for taking 
out the work 

A jig construction adapted to drilling holes on an 
angle is illustrated in Fig. 18-37. In this case, a 


not it i h 

Fig. 18-34- A drill jig provided with rockers to facilitate reversing its position. 




Ch. 18 

: I s 


(_ * 



i O O i 

Fig. 18-35, Vise with a jig plate. 

swivel vise is fitted with a plate A, which can be set 
at the proper angle in relation to the base B by 
swinging the vise around axis C. 

Jigs for Multiple Spindle Drilling 

Multiple-spindle drilling machines and multiple- 
spindle drill heads mounted on single -spindle drill 
presses have their drills already set in the required 
pattern. Some of these machines and drill heads 
carry their own drill jig, when they are provided 
with drill bushings mounted in the manner shown in 
Fig. 14-16; they require only a work-positioning 

When no such devices are present, the multiple 
spindle machines and drill heads need a drill jig for 
locating and clamping the part and for guiding and 
supporting the drills. This is particularly important 
as the drills, in many cases, are small and long. 

These machines are used in mass production, and 
fast manipulation of the drill jig is of the greatest im- 
portance. Depending on the number of drills in oper- 
ation, the load on the drill jig is usually quite large. 

Indexing Drill Jigs 

Indexing devices are described in Chapter 6, Design 
of Locating Components, and two indexing drill jigs 

Fig. 18-36. A viae with V-blocks and a removable jig plate. 

Ch. 18 



Fig. 18-37. A tilting vise with a jig attachment. 

are shown in Figs. 6-35 and 6-36, A special case of 
an indexing drill jig where the part is its own index- 
ing plate, is shown in Fig. 1 8-38, 

This jig was used for drilling dial plates of the form 
employed on automatic feed mechanisms for power 
presses. These dial plates had the center hole bored 
and the notches milled to suit the locating plungers 
on the power presses, but the holes had to be drilled 
later because they were located with reference to the 
particular presses on which the dials were used. 
Before using the drill jig it was necessary to make 
center punches to fit the punch -blocks on the differ- 

ent power presses and also to fit bushing A in the 
jig. Each dial plate fl was then put on its bed and 
the press was set in the usual way, care being taken 
to have the locking device fit properly in one of the 
notches. The center punch was then mounted in 
the punch-block and one prick-punch mark was 
made on the dial in proper relation to one of the 
notches. The dial plate was next placed on the 
table of a drill press and the center punch was set in 
the chuck in the drill spindle so that the prick-punch 
mark on the dial could be lined up with the spindle. 
The plate was then strapped to the table and stud C 

Fig. 18-38. An indexing drill jig. 



Ch. 18 

driven into the center hole. The top of stud C 
was machined to fit the pivot hole in the arm D of 
the jig. 

The next step was lining up the bushing A of the 
fixture with the center punch in the drill spindle. 
The bushing was made adjustable relative to the 
center C, about which the arm swung, so that it 
could be set in the required position before clamp- 
ing the binding bolt. The bushing was located in 
proper relation to the notches in the dial plate 
by means of the locking pawl E, and the eccentric 
screw F adjusted the position of the pawl relative to 
the arm D of the jig. The pawl was held in the 
proper notch in the dial by spring G which was 
mounted on pins H and /; and stud J was used to 
hold the arm of the fixture true with the face of the 
dial plate. It will be evident that after this setting 
had been made, bushing^ would be located directly 
over the center punch mark which was made on the 
dial plate while the prick -punch was mounted in the 
punch-block of the power press. The hole could then 
be drilled in the dial plate, after which successive 
holes were drilled by simply swinging the dial around 
pivot C, and locking it for drilling each hole by drop- 
ping pawl E into successive notches in the dial plate. 

Miscellaneous Drill Jigs 

Interesting and characteristic types of drill jigs 
have been used as examples in previous chapters. 
The jig shown in Fig. 1 1-2 is a clear-cut example of 

a box jig with some good design details. Drill jigs 
for small parts that require the hole exactly in the 
center of the part are shown in Figs. 9-17, 9-18, 
9-19, and 9-20. A simple, inexpensive, and very 
versatile jig is the bracket type of drill jig shown in 
Figs. 10-37 and 10-38. 

Where a large number of parallel holes (six or more) 
all require the use of one or several slip bushings, 
the time required for inserting and removing the 
bushings becomes substantial. Considerable time can 
be saved by using a bushing holder, a plate carrying 
all slip bushings of the same type, where all bushings 
are removed and inserted in one manual operation. 
The cost of a bushing plate is negligible. 

The smaller the part, the more important it is that 
the operation is fast, and much can be gained by 
using sophisticated, yet simple, devices for closing 
the jig and clamping the part. The drill jig shown in 
Fig. 18-39 is a good example of the simplicity of 
design that can be attained by using the bayonet- 
lock type of clamp. A clamp of this type also 
keeps the loading time down to the minimum re- 
quired for economical production. The jig is com- 
posed of only six pieces -the body, clamp, pin, and 
three bushings. The side of the body opposite the 
drill bushing for the angular hole in the work piece is 
machined at an angle of 90 degrees to the axis of the 
bushing hole to serve as a base while drilling the 
hole. This arrangement eliminates the necessity of 
providing a separate angle-block. 


Fig. 18-39. A jig with quick-acting bayonet clamp. 

Ch. 18 



The jig is designed for drilling the angular hole at A 
and the two holes B and C at opposite sides of the 
work. The workpiece comprises a subassembly of 
a high-pressure valve and stud for a sensitive air- 
control valve which is part of an air brake. 

The body of the jig is bored to a slip fit for the 
workpiece. The opposite end of the jig is flattened 
on both sides of the center to meet the bottom of 
this bore. This provides openings at D and E for 
the escape of chips. A clearance hole F is also 
drilled through this end to clear the stud in the 
workpiece. The three drill bushings are pressed into 
holes that are accurately positioned in the jig body. 
The clamp is a slip fit for the hole in the body, 
which is made larger than the locating bore for the 
work, so that the jig will be easier to load. The top 
of the hole in the body and the end of the clamp 
are chamfered to facilitate insertion of the clamp. 
Bayonet slots G on the sides of the body are made 
a slip fit foT the pin pressed into the clamp. These 
slots have a radius bend and a lateral section which 
permit the pin to be given a clockwise turn. These 
lateral ends of the slots are machined at an angle of 
about 95 degrees to form cam surfaces which give 
the bayonet lock its clamping action against the 
workpiece. The lateral slot on one side extends in the 
opposite direction from that on the other side. The 
work should be clamped when the pin is at about the 
middle of the lateral part of the slots. 

The length and diameter of the hub on the end of 
the clamp are such that the hub clears the plane of 
the flat, on the side of the body on which the jig 
rests when drilling the angular hole. This flat pro- 
vides sufficient surface beyond the center line of the 
bushings to permit drilling one side hole, and the 
angular hole, without causing the jig to tip. Because 
it is necessary to have a small hub at the end of the 
clamp, a hexagon socket is machined in it to fit an 
Allen wrench, so that the clamp can be easily tight- 
ened or loosened. 

The jig shown in Fig. 18-40 has an equalizing 
member for the combined clamping and closing 
operation, and is used for drilling and tapping stud 
A, which is made from 1/4 X 1/4-inch (6 X 6-mm) 
cold-drawn steel. The end of the stud enters hole B 
in the locating block, and this hole is milled to 
provide clearance for the head of the stud. The work 
rests on the drill bushing which is slightly counter- 
bored to provide clearance for the tap. The interest- 
ing feature of the jig is that the cover and clamping 
mechanism are both secured by the same knob; 
clamp C is swung around its pivot to hold the stud 
securely in place when the knob is screwed down, 
and the same operation tightens the cover. This 

principle permits fast opening and closing of the jig, 
and can be employed on jigs and fixtures used for 
holding a great variety of parts. 

Fig. 18-40. A jig with an equalizing member for combined 
clamping and closing. 

Occasionally a hole must be drilled in the inter- 
face between two parts that are fitted together, and 
a pin is driven into the hole to act as a lock or key. 
In job shop work this is done at assembly; under 
manufacturing conditions it is preferable to perform 
these operations on the two parts separately, prior to 
assembly. To drill such a "half" hole, it is usually 
necessary to plug up the hole in the work in some 
way that will back up the side of the drill that is 
not cutting. This is accomplished, as shown in Fig. 
1 8-41, by means of a hardened stud A with a semi- 
cylindrical groove that matches half of the surface 
of the drill. The stud has a push fit in the work and 
backs up the drill during the drilling operation. An 
angle iron or plate. B, is attached to stud A and held 
in position by bolt C; plate B is also doweled in place. 
A hole is drilled in this angle iron to receive bushing 
D, which guides the drill in the usual manner. The 
remainder of the jig consists of the key E which 
locks the jig in place on the work. 

In using this tool, key E is pulled back, clear of the 
work, and stud A, which carries the angle iron, is 
pushed into the hole until the stud moves up against 
the shoulder of the work. By pushing up tapered 
key E until it binds on the flat of the work then tap- 
ping it lightly, the jig is held securely in place. 



Ch. 18 

Fig. 18-41. A jig for drilling half holes. 

When drilling the hole, the work is set up on end 
on the drill press table and the drill is then fed 
through the bushing in the usual manner, the bush- 
ing holding the drill in position until it starts to 
cut. As the drill is fed down, there is a tendency to 
force it away from the work, but this tendency is 
resisted by the hardened stud A so that the half 
hole is drilled parallel with the axis of the work. 
Even with a drill jig, this is a difficult operation. 
When drilling with an ordinary twist drill there is a 
tendency for the drill to "hog in," which is apt to 
result in the tool breaking, For this reason, a drill 
with zero rake angle is recommended; either a 
straight-fluted or farmer's drill or an ordinary twist 
drill ground in such a way that it has no rake, 

A jig that clamps quickly, and with spring pressure, 
is shown in Fig. 18-42. It is a representative ex- 
ample of a homemade pump jig. The jig is shown 
empty. A drill bushing A is mounted in a movable 

traverse which, by spring pressure, is forced down 
and clamps on the work. One locator is installed 
under the bushing and another locator B is carried 
on a bracket C. To unclamp and open the jig, the 
operating handle D, is depressed. It swings around 
pivots E and lifts rods F which in turn lift the tra- 
verse with the bushing against springs G. 

Tooling for N/C Drilling Machines 

In N/C drilling machines, the locating of the spin- 
dle is automatically controlled from a tape- or card- 
operated electronic control unit. Consequently, 
these machines do not require the conventional type 
of drill jig with bushings for locating the drill rela- 
tive to the work. Most, but not all, N/C drilling 

Fig. 18-42. A homemade pump jig. 

Courtesy of Heald Machine Div., Cincinnati Milacron Inc. 

Fig. 18-43. A fixture with hand-operated clamps for N/C 

drilling and boring of holes in a circular part. 




machines are used for job shop or short run opera- 
tions. This is a highly competitive type of business, 
tooling expenses must be kept low and the tooling 
is reduced to the simplest possible type of fixture 
for the sole purpose of supporting and clamping the 
work. Quite often, the job is done entirely without 
special tooling. 

The clamps used are, as a general rule, hand- 
operated strap clamps. Another general require- 
ment is low construction height to avoid collision 
when the table sweeps back and forth under the 
spindle. Two characteristic examples are shown in 
Figs. 18-43 and 18-44. In each case, the fixture 
consists essentially of an aluminum tooling plate 
with a few accessories. In Fig. 18-43, the part is 
centered in a circular recess in the plate which also 
carries four studs for the strap clamps. In Fig. 
18-44, the part is located against locating pins (vis- 
ible on the right-hand side) and clamped with strap 
clamps having knurled-head hand knobs. 

Where N/C drilling is applied to large-volume pro- 
duction, it becomes economically feasible to use 
quick-acting and more sophisticated clamping de- 
vices. A typical example is the drilling of circuit 
boards. These boards are manufactured in fairly 

large quantities, in widely different sizes and are 
stack drilled. For these reasons, the clamping de- 
vices must be horizontally and vertically adjustable 
and quick acting, as shown in Fig. 18-45. The 
carriage provides the horizontal adjustment for vary- 
ing board sizes, and the air clamp cylinder has suf- 
ficient length of travel to accommodate stacks of 

SIZE Z" « 3" TO 12" x 24" 



Courtesy of ileald Machine Div., Cincinnati Milacron Inc. 

Fig. 18-45. Air-operated and adjustable clamping device 

for a fixture for N/C drilling of circuit boards. 

Courtesy of Heaid Machine Div., Cincinnati Milacron Inc. 
Fig. 18-44. A fixture with hand-operated ciamps for N/C drilling of a rectangular casting. 



Ch. 18 

boards of varying height. Any automatic clamping 
device should comprise a safety feature against acci- 
dent or damage in case of failure of the operating 
pressure. In the present case, this is accomplished 
by a pressure switch. Should the air pressure drop, 
the machine will stop. In other cases it may be 
done by means of a toggle clamp or a self-locking 
eccentric, cam, or wedge. 

N/C drill fixtures, as well as other N/C fixtures, 
usually incorporate a locating device to establish 
one spindle position in relation to a reference point 
or surface on the part. This position serves as the 
correct starting point for the programmed sequence 
of all following spindle locations. 

With the absence of bushings, the drill point has 
no support and guidance as it meets the surface of 
the part, and it may "walk" a little before it enters 
the metal. The position accuracy is therefore less 
than with the use of bushings. If the position 
tolerance is ±0.010 inch ( + 0.25 mm), a total posi- 
tion variation of 0.020 inch (0.5 mm) or less, it is 
necessary first to spot drill with a short and rigid 
center drill, or to use a twist drill with a spiral 
ground point. 

A drastic and illustrative example of the saving in 
fixture cost that may be realized by replacing con- 

ventional drilling machine equipment with an N/C 
drilling machine is shown in Fig. 18-46. The part is 
a casting for a fuel pump housing, and the fixture 
required for N/C drilling of this part consists of a 
base plate, an angle plate, a bolster plate, and a 
clamping stud with nut. The total cost of this fix- 
ture was less than S 600.00, while the cost of the 
various drill jigs required for conventional drilling 
was over S 5000. 00. 

An additional case is the instrument frame shown 
in Fig. 18-47a, which requires considerable machin- 
ing with end mills, and the drilling of a large number 

| --h \4 J5 

I : I I I : j ' ti. 

I =• 



Fuel-pump housing Numerical control holding fixture 

Courtesy ofMetaiworking Magazine 
Fig. 18-46. An example of simplification in fixture design 
that can be realized with the use of N/C 
drilling equipment. 

Courtesy of Cincinnati Milacron Inc. 
Fig. 18-47. a. An instrument frame to be N/C machined, b. The instrument frame in position on the fixture for the N/C 
machining operations. 


of holes some of which also require tapping. These toggle clamps, and two circular locators matching the 

operations are performed on an N/C machine tool contour within the three small lugs inside the two 

in the setup shown in Fig. 18-47b. The fixture is circular openings. By these simple means, the part 

built up on a tooling plate as the base and consists is supported, located, and clamped. The clamps 

essentially of rectangular blocks, bolted to the base. which are here shown in their retracted positions, 

They carry the part, one cam -operated clamp, two present a low profile relative to the part. 



Design Studies II — Milling Fixtures 

Milling operations are characterized by large, 
periodically varying cutting forces, producing a large 
volume of chips, usually of small size. The tool 
may be a single milling cutter or a set of gutters. 
The operation is normally completed in one pass of 
the cutter. In most operations the path of the 
milling cutter relative to the work is a straight line. 
However, the fixture may be clamped on a revolving 
table for cutting an arc of a circle, or some other 
curve may be cut as in tracer controlled and contour 

To meet these conditions, milling fixtures must be 
sturdy, with relatively large locating and supporting 
areas and very strong clamps. Wherever possible, 
cutting tool pressure is taken up by positive stops, 
rather than by friction, which may fail under vi- 
bration. To reduce loading and unloading time, 
fixtures for volume production are equipped with 
pneumatically or hydraulically operated clamps. 
Hydraulic operation is preferred, since oil has less 
inherent elasticity than air, and because hydraulic 
actuators can be made with smaller dimensions for 
the same clamping force. Pneumatic and hydraulic 
clamping devices must have a safety locking feature, 
as explained in Chapter 1 S, page 242, to prevent 
accidents in case of a power failure. 

In principle, a milling fixture is a box, preferably 
of open design, i.e., open at the top or at one side 
for giving easy access to the cutter for reaching the 
surface to be machined, and also to the locating 
areas for cleaning away chips. These two cases are 
illustrated in Figs. 1-2 and 1-3, which show most of 
the locating, supporting, and clamping components 
that are typical for milling fixtures. Attention is 
drawn to the tool setting block (7 in Fig, 1-2) 
with which the milling cutter is positioned for the 
correct location of the cut. The gage pin in Fig. 1-3 
has the same function. 

These features are clearly seen in the fixture 
shown in Fig. 19-1. The fixture base is mounted 

Courtesy of Monarch Machine Tool Co. 
Fig, 19-1. A typical milling fixture mounted vertically on 
an angle plate. 

vertically by means of an angle plate, as the face- 
milling operation is done on a horizontal milling 
machine. For the same operation on a vertical 
milling machine, the fixture would be mounted 
directly on the machine table. 

The milling machine vise with detachable jaws 
or inserts, contoured to fit the part, provides many 
opportunities for the design of inexpensive milling 
fixtures. Design details are given in Chapter 10, 
Clamping Elements. In production milling it is often 
economically advantageous to use more than one 
fixture. The combined length of the run-in and 
run-out distance for the milling cutter is usually 
quite significant relative to the net length of the 
machined surface, as illustrated in Fig. I 9-2. A con- 
siderable saving in operating time is accomplished 
by string milling, where a number of identical 
milling fixtures are mounted as closely together as 


Ch. 19 






K r 






Fig. 19-2. Milling of a single part, and string milling, show- 
ing the significance of cutter run-in and cutter 

possible, in a line, on a common base. Duplex, mill- 
ing, i.e., milling of two parts in one operation, is a 
common as well as a profitable operation. Multiple- 
spindle milling machines naturally require multiple 
milling fixtures and an example is shown in Fig. 1 9-3. 

Courtesy of Cintimatic Div. of Cincinnati Mttacron Inc. 
Fig. 19-3. Milling with a three-spindle milling machine, 
utilizing three milling fixtures mounted on an 
angle plate base. 

The systematic fixture design procedure outlined in 
Chapter 3, and exemplified in Chapter 18, will now 
be applied to three widely different cases of milling 
fixtures with commercial components used wher- 
ever possible. 

Case 11. Design a fixture for the milling of the sur- 
faces on the back side of the part shown in 
Fig. 1 9-4, the housing for the lead screw drive on a 
medium-size engine lathe. 

The part is a gray iron casting, weighing 45 pounds 
(20 kg). In comparison, the complete fixture weighs 
empty, 95 pounds (43 kg). The system of surfaces 
to be machined on the back side consists of an 
upper and lower longitudinal recess extending over 
the entire length of the part, and recesses on two 
bearing parts at and near the right-hand-end of the 
part. They can be gang milled in one pass with the 
set of milling cutters shown in Fig. 19-4. This 
operation is selected as the first step because it con- 
stitutes the major single operation on the part, and 
it provides excellent locating surfaces for all sub- 
sequent procedures. The design develops as follows: 

1. For the first operation, the part must be 
located and clamped entirely on raw cast surfaces. 
This presents no problem because the part has a 
regular geometry, with large flat surfaces at right 
angles to each other, and, in addition, there are 
three bosses in a triangular pattern on the front. 
Furthermore, all surfaces to he considered for lo- 
cating, supporting, and clamping are free of casting 
contaminations, such as mismatch and flash. In fact, 
the part offers the possibility of a classical applica- 
tion of the 3-2-1 principle, using hardened spherical 
buttons as the locating points. The locators are 
shown in Fig, 19-5 and are identified by®. 
Three buttons carry the part on the three bosses; 
two buttons on one side align the part, and one 
button on the end locates it endwise. 

2. The part is clamped against the locators by 
three 5/8-1 1 UNC (16 X 2mm) bolts, © , arranged 
opposite the side and end locators. The clamping 
bolts are inclined 5 degrees, so that they aim below 
the side and end locators and force the part down on 
the three base locators. 

3. A critical examination at this stage shows that 
the part is not fully stable. If large forces are 
applied outside the locator triangle and near the 
two corners, the part may tilt over one side of the 
base triangle by slipping slightly under the clamping 
bolts. To prevent this, one or several intermediate 
supports are needed. Applying two more base sup- 
ports near the corners will provide the classical, 
rectangular, and very efficient, support pattern; 
however, such supports must be individually adjust- 
able, as they must act on raw surfaces. Screw jacks 
may be ruled out since they would not be easily 
accessible. Spring loaded jacks are quite long and 
would therefore raise the part a considerable dis- 
tance above the machine table, thus substantially 



Ch. 19 



.12 ffff TYP 






. ;00S 







Fig. 19-4. A lead screw drive housing for a lathe. 

sacrificing rigidity in the setup. Endwise, there is no 
such dimensional limitation, and a spring loaded 
jack is mounted symmetrically with, and parallel to, 

Fig. 19-5. Components for locating, clamping, supporting, 
and cutter guidance. The components are num- 
bered to indicate the corresponding step in the 
systematic design procedure. 

the end locator. The jack applies itself to the sur- 
face of the part by spring pressure, and is then 
secured by the hand knob locking screw. These 
parts are identified by Q) , 

4. The relative location of the individual milled 
surfaces is defined by the milling cutter assembly, 
and the cutter guide has to locate only one corner 
of one cutter relative to the part. The cutter 
guide, (3) , is a hardened steel plug, provided with 
a 90-degree step with horizontal and vertical guid- 
ing surfaces. To avoid wear on the precision 
surfaces, a 0.120-inch (3.05-mm>thick feeler gage 
is laid against the cutter guide when the cutter 
is adjusted. 

5. The design of the complete fixture, as shown 
in Fig. 19-6, follows almost automatically from the 
pattern of the previously described components. It 
is a rectangular box with ribs beneath the bottom, 
and heavy section uprights on the sides for added 
rigidity. It lends itself well to casting and does not 
require much core work. The base locators are 

Ch. 19 






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-f t„-.Op I ^- Mil L I BfAMJTSIWfW 1 




. SPACE FOR __ . ,j,bjl nfl» 

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.010 REE ^-l_-.2»-*«tf, 

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Fig. 19-6. The complete milling fixture. 

lifted from the bottom on bosses to facilitate ma- 
chining and to ensure that they stay cleat of chips. 
The side walls have windows for chip removal. The 
fixture is bolted to the machine table with four 
T-bolts, and is aligned with two keys in one T-slot. 
The closest tolerances on the part are those con- 
trolled by the mounting of the milling cutters on the 

arbor; the remaining tolerances are quite liberal. 
Therefore, there is no place where the conventional 
toolmaker's tolerance of 0.001 inch (0.025 mm) is 
really needed, and all tolerances on the fixture 
except those for press fits) are 0.002 inch (0.050 
mm) or multiples thereof. 




L- 325W - 


Fig. 19-7. A slide base for a special machine toot. 



Ch. 19 

Case 12. Design a fixture for the milling of the 
upper and side surfaces of the part shown in 
Fig, 19-7. 

Detailed drawings of the fixture body and the 
clamp strap are shown in Figs, 19-g and 19-9, 
while the complete fixture is shown in Fig 19-10. 

clamping surface for the subsequent milling opera- 
tions of the upper surfaces. The outline of the 
milling cutter assembly for this operation is shown 
in Fig. 19-7. The design proceeds as follows: 

1. With some modifications, the 3-2-1 principle 
can be applied. The fixture body has a large ma- 


I *J 10010/ 

« — 4— iS&M t~-TW3 K . 

mamuL-eiurctsi iron 

Tig. 19-8. Detailed drawing of the fixture body. 

The part is a gray iron casting, weighing 13 pounds 
<6 kg). In comparison, the complete fixture, empty, 
weighs 52 pounds (24 kg). The part is to be used as 
the base for a small slide in a special machine tool 
and the surfaces to be milled form the guideways for 
the slide. 

The first operation to be performed on the part is, 
naturally, the machining of the bottom surface. This 
operation requires no fixture because the size and 
shape of the part permit it to be securely clamped 
in the milling machine vise. And once the bottom 
is machined, it offers an excellent locating and 


■usacr ecxxim misolggatk 



1 -"-'.&' 

UHDIHlmiOHL i> MWI .15101 S 

*F-flv™w r OHMfflj mmmii nit 

MAT£WAl ■■ AW 4T40$T{EL HT TOIsajjOOPii 
ALL su&tt£S T^ 

Fjg. 1 9-9. Detailed drawing of the clamp strap. 

chined flat surface to receive the machined bottom 
surface of the part. This is the equivalent of the 
first three locators. Dimensions and tolerances for 
the part indicate that a considerable degree of 
symmetry is required, which includes the two un- 
machined edges of the base flanges. It is therefore 
necessary to provide a system of centralizcrs, acting 
on the side edges of the flanges. In the present 
case, this is accomplished by an unconventional de- 
sign of the clamp straps. Each strap is fork shaped 
and the fork prongs have downward projecting 
strips arranged in a V-shape, as seen in Fig. 1 9-9. 

These two V's act on each two corners of the 
flanges. To confine the clamp straps to symmetrical 
positions, and still permit longitudinal movement, 
each strap has a longitudinal slot with a sliding fit 
around the clamp stud, and a tail which is guided, 
also with a sliding fit, in the rest block which is in- 
tegral with the fixture body. When the two V's are 
brought into contact with the four corners of the 
flanges on the part, the part is confined to the re- 
quired symmetrical, posit ion, but so far, longitudinal 
motion is still possible. To finally eliminate this 
motion, one end stop, in the form of a round rest 
button, is mounted with a press fit in the fixture 
body. The locating components, thus described, are 
identified by (T) in Fig. 19-10. 

2. The part is clamped against the bottom locat- 
ing surface by the two fork-shaped clam p straps and 
two 3/4-10 UNC (20 X 2.5 mm) clamp studs with 

Ch. 19 




ciampmvbA tointe-tessoi- 

CU7 T04-IKtri£MGm 


i W»aii7s) — — sphih waitiiASBir 
fori* -m srus 




HT. 70S*- 56 ROCKWELL C 

iftteoijtMtiur, i~to- 

1106-25425) * 





Fig. 19-10. The complete milling fixture for the part shown in Fig, 19-7. 


IABB- -H76 




nuts and spherical washers. Each strap clamps on 
the part at two points, and is reacted by the rest 
block under its tail end. The design is such that the 
stud is located approximately in the center of 
gravity for the three pressure points, so that the 
total clamping force is distributed quite evenly on 
these three points. When in operation, the strap 
opposite the end stop is moved forward so that the 
part is brought into contact with the end stop. The 
force from the milling cutter acts in the di- 
rection. In view of these facts, and the substantial 
vertical clamping forces exerted by the two straps, 
no additional longitudinal clamping means are need- 
ed. The clamping components are identified by (2) 
in Fig. 19-10. Note the details of the design of the 
tail end which permits the strap to tilt and adjust 
itself to any unevenness in thickness of the flanges 
without binding of the tail end. Had there been 
lifting springs under the clamp straps, the operator 
would have found them convenient. However, the 
available space is too narrow to allow the installa- 
tion of such springs. 

3, Because of the rigidity of the part, and the 
uniform support which it receives from the base, 
there is no need for any intermediate supports. 

4. As in Case 1 1, the relative location of the 
individual milled surfaces is defined by the milling 
cutter assembly, and the cutter guide has to locate 
the cutter assembly in the vertical and horizontal 
directions. The cutter 'guide, (4) , in Fig. 19-10, 
is a hardened plug of tool steel, mounted with a 
press fit in an extension of the rest block, at that 
end of the fixture which is opposite the end stop. 
The cutter guide has horizontal and vertical locating 
surfaces, with dimensions that allow the use of a 

.120-inch (3.05-mm) feeler gage when setting the 
cutter. In this case, no detailed drawing is provided 
of the cutter guide, but it is recommended that the 
reader, as an exercise, make a complete drawing of 
the cutter guide (including grinding clearances, if 
needed), and calculate the required tolerances. 

5. The design of the fixture body, shown in 
detail in Fig. 19-8 and identified by (?) in Fig. 19- 
10, follows almost automatically from the previous 
discussion. Essentially, it consists of a heavy base 
with the locating surface for the part, two rest 
blocks at the ends, and slots for the keys and T- bolts 
which align and secure it to the machine table. 
All tolerances, except those for the cutter guide 
(to be calculated in the recommended exercise), are 
quite liberal. There are no closed spaces and no 
chip cleaning problems. The design lends itself well 
to casting and requires no core work. However, it 
is equally well suited for welded construction. 


==j=l '■'■»«- 

1 *«3I 

v^ i ^r 




r Mt.03 

Hi TEPtfAl.- GflW CASF WOrV 

Hg. 19-11. A bracket with two bearing bosses. 



Ch. 19 


ZSQ&jOQQ fyp 

2,00 1 


30 a* 

mpi in typ 

J-- SOU jVC - 75fl 


. 7i CIA 


Fig. 19-12. Detailed drawing of the fixture body. 

Case 13. Design a fixture for the milling of the 
base surface of the bracket with two bearing bosses 
as shown in Fig. 19-11. 

Detailed drawings of the fixture body, a V-block 
locator, and the clamp straps, are shown in Figs. 
19-12 and 19-13, while the complete fixture is 
shown in Fig. 19-14. The part, a gray iron casting, is 
a bracket with two bearing bosses, and weighs 28 
pounds (13 kg). In comparison, the complete 
fixture, empty, weighs 136 pounds (62 kg). 

The part comes unmachined, and it is natural to 
select the machining of the base surface as the first 
operation since this provides excellent conditions 
for fixturing the following operations. This choice 
is not without its problems, since the part has no 
other flat surfaces on which it could be located and 
clamped for the first operation. However, the bear- 
ing holes are cored out to 1 3/8-inch (35 mm)- di- 
ameter, so that the part can be well clamped in the 
cored holes while it is located and carried on the 
cylindrical outer surfaces of the bosses. This 
method of locating assumes that there is no parting 
plane with its inherent danger of mismatch across 
the bosses. Depending on the type of milling 
machine to be used (horizontal or vertical), the 
milling cutter is either a plain milling cutter with 
helical teeth, slightly longer than the width of the 

part, or a face milling cutter with a diameter sub- 
stantially greater than the width of the part. It is 
left to the reader, as an exercise, to make a recom- 
mendation for the diameter of the face milling 
cutter. When it comes to the detailed planning of 
the operation, the direction of the cutter tooth 
helix, or the teeth in the face milling cutter, to- 
gether with the rotation of the milling machine 
spindle, must be such that the side component of 

twEftm rtisi toio met ' fiAO R £r 


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ai t sv*FAcejJ~mAs /.or ft? jo JH-*-*z L 


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Fig. 19-13. Detailed drawing of the V-block locator and 
the clamp strap. 

Ch. 19 





g M^Tti-i Qx, — mm 

— /—i i*b 


iwp aw iuau 




grwng ro height wm-ffasG 

clamp jroa J -/o iAve-4osm „ 

HEX NtlT-i 

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/ $7Vt>it£>6-2i?W ~ — 


2f LOW ,'iO5-:-0C9J, 

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400 REF 






*■£»! W-A--JtS.H.CAP 


Fig. 19-14. The complete milling fixture for the part shown in Fig. 19-11. 

the cutting force is acting against the side stop In the 
fixture. The design proceeds as follows: 

I, The part can be located and carried with the 
two bosses supported in a double V-block, shown in 
detail in Fig. 19-13. The, 3- 2-1 principle is not 
directly applicable, hut the support in the V-block 
eliminates four degrees of freedom; namely, two in 
the vertical direction and two in the side direction. 
At the same time, the bearing axis is centered. The 
part can still rotate in the V's around this axis, and 
it can slide longitudinally, thus having two degrees 
of freedom. These two freedoms are now eliminated 
by the addition of a side stop and an end stop. The 
side stop is formed by a 5/8-1 1 UNC (16X2 mm) 
hexagonal head screw, acting against the side of the 
base. The screw is locked in position by a jam nut. 
In this way, the position of the side stop can be 
adjusted when necessary, as, for example, if a batch 
of castings should fall outside of dimensional toler- 
ances. Only one side locator is required, as the 
direction of the bearing axis is already defined by 
the V's. The end stop is a spherical button. The 
locating components are identified by (T) in 
Fig. 19-14. 

2. The part is clamped down into the V-block by 
two finger-type clamp straps, shown in detail in 
Fig. 19-13, and two 3/4-10 UNC (20 X 2.5 mm) 
clamp studs with nuts and spherical washers. The 
straps have relief grooves cut across at a distance 
of 1/2 inch from either end. These details allow 
the straps to seek the lowest point in the cored 
holes, as the nuts are tightened, and to adjust 
themselves to slight dimensional variations in the 
parts. Lifting springs around the studs are provided 
to hold the clamps up when in the retracted posi- 
tion, for the convenience of the operator. A 
5/8-1 1 UNC ( 16 X 2 mm) hexagonal head-clamping 
screw is provided to clamp the part against the side 
stop. A 5/8-11 UNC (16 X 2 mm) hand-knob 
screw is provided to clamp the part against the end 
stop. Here a hand-operated screw is preferred be- 
cause it provides more "feel" in clamping, than 
a hexagonal screw operated with a wrench. In 
clamping, the part will be laid down into the V's, and 
manually held against the side and end stops, while 
the hand- knob screw is applied. Subsequently, the 
hexagonal head screws are tightened. The clamping 
components are identified by (2) in Fig, 19-14. 



Ch. 19 

3. The area to be machined is quite wide and a 
large cutting force is generated, including a sub- 
stantial side-force component. It is therefore neces- 
sary to -supplement the side slop and its mating 
clamping screw with additional supporting means, 
equivalent to what is known as "intermediate 
supports." They must be hand operated, to be 
applied with proper "feel" so as not to strain the 
part out of the position in which it is held by the 
previously applied clamps. These additional supports 
comprise two more 5/8-i 1 UNC (16X2 mm) hand- 
knob screws, (3) in Fig. 19-14, arranged opposite 
each other. 

4. The cutter can be sidewise located by sight, 
since it is visibly wider than the part. The vertical 
Locating of the cutter is done with a cutter guide 
consisting of a round rest button, installed with a 
press fit in one of the four uprights that form 
part of the fixture body. This particular upright is 
1/8 inch (3 mm) higher than the three other up- 
rights and is machined at the top. An allowance of 
.120 inch (3.05 mm) is provided for the feeler gage 
used in the setting of the cutter. A recommended 
exercise, is to calculate the tolerance on the height 
dimension of the cutter guide, @ in Fig. 19-14. 

5. The design of the fixture body, shown in 
detail in Fig. 19-12 and identified by (3) in Fig. 
19-14, follows almost automatically from the posi- 
tions of the locating and clamping components. It 
consists essentially of a heavy base, four uprights, 
and two upper cross bars, carried by the uprights. 
The base has the locating surface for the V- block, 
two rest blocks at the ends, and slots for the keys 
and T-bolts which align and secure it to the ma- 
chine table. The four uprights carry the side stop 
and its clamping screw, and the two additional 
hand-knob screws. One upright carries the cutter 
guide. The two uprights in either side are connected 
by a 5/8-inch (16-mm>thick wall for added rigidity. 


The two uprights at either end are connected by 
two cross bars which carry, respectively, the end 
stop and its hand-knob clamp screw. 

The fixture body is designed as a casting. It 
requires some core work, but the core is of a regular 
and simple geometry and presents no technical or 
economic problems. However, this fixture body 


- fix tunc 


Fig. 19-15. a. A milling fixture with manual clamping, b. 
cylinder head), milling cutters, locating points, arid 

_(— u-rmr—i 


Courtesy of Cincinnati Milacron Inc. 
A line drawing of the same fixture showing workplace (a 
the gage block. 

Ch. 19 



Courtesy of Cincinnati Milacron Inc. 
Hg, 1 9-16. A milling fixture clamped on the machine tool 
table by means of end clamps. 

Courtesy of Cincinnati Milacron Inc. 
Fig. 19-17. A milting fixture made by modifying the jaws 
of a swiveling milling machine vise. 

lends itself equally well to welded construction or 
to a combination of welded and built-up con- 
struction. While the V- block is a precision part, all 
tolerances within the fixture body itself are quite 
liberal. The only dimension that requires a some- 
what close tolerance is the height of the cutter guide. 

Typical Milling Fixtures 

A typical milling fixture is shown in Fig. 19-15, a 
and b. The work is a cylinder head, outlined in Fig. 
1915b. It has a previously machined surface and is 
located by this surface on five blocks, each with two 
narrow bearing surfaces. The clamps are hand- 
operated clamps of normal design. Further locating 
is done by two locating pins fitting into two holes 
In the block {tooling holes). The cutter setting 
gage is located on a bracket on one side of the fix- 
ture. This fixture is aligned with the muling ma- 
chine spindle by means of two keys, called "tongue 
strips," one at each end, which fit into a T-sIot in 
the milling machine table. The fixture also has two 
ordinary slots at each end for the clamping bolts. 
While this is a widely used practice, it may be less 
practical when a fixture is expected to be used on 
several milling machines as the spacing of the T-slots 
may be different on different machines. It is, in 
this sense, more practical to use end clamps, as 
shown in Fig. 19-16. 

The milling machine vise with modified jaws 
provides many opportunities for the design of inex- 
pensive milling fixtures. An example is shown in 
Fig. 19-17. String milling is used extensively, and 
some examples are shown in Fig, 19-18. 

Duplex milling, that is, the milling of two parts in 
one operation, is also a common and profitable 

Contour and Profile Milling Fixtures 

Contour or profile milling fixtures are used on 
profiling or contour milling machines. These fix- 
tures are basically similar to other fixtures; how- 
ever, a distinctive characteristic of this type of 

Courtesy of Technological Institute, Copenhagen 
Fig. 19-18. Examples of string milling. 



Ch. 19 

Courtesy of Cincin na ti Milacron In c. 
Fig, 19-19. Milling fixtures and template bracket for profile milling cylinder heads. Fixtures and template are trunnion 


equipment is that it must provide for a bracket for 
holding the cam or template which controls the 
operation of the machine. The fixture must also 
have setting gages for aligning the fixture and the 
template bracket with the machine and the tracer 
spindles, as explained in Chapter 13, Cutter Guides. 
Two or three spindles are frequently found in 
tracer controlled and contour milling machines. 
They require the corresponding number of identical 
fixtures for simultaneous milling of several parts. An 
example is shown in Fig. 19-19. Here, two fixtures 
are used to hold two cylinder heads. The template 
bracket is a trunnion-mounted box holding tem- 
plates for several different operations. This box 
indexes between operations so that only one tem- 
plate at a time is brought into the active position. 
In this case, various operations require different 



l ; ig. 19-20. Part to be milled on sides A, B, and C. 

angular positions of the part, therefore the fixtures 
are built as trunnion bases with cradles. 

Adjustable and Movable Milling Fixtures 

Milling fixtures are made adjustable or movable for 
several reasons. An adjustable fixture designed for 
the milling of the nonparalle! sides of the block 
shown in Fig. 19-20, is illustrated in Fig. 19-21. 
Three operations are involved; the parallel sides A 
are milled by means of the straddle cutters, and the 
two sides B and C are then milled in two subsequent 
operations. The three operations are all performed 
without requiring more than one setting of the work. 
The block is cut off from bar stock, and drilled and 
counterbored to receive two fillister-head screws 
which hold it in place on the machine of which it 
forms a part. These holes are also utilized for hold- 
ing the block in position on the fixture. 

The milling fixture consists of an upper plate A, 
which is pivoted on stud B. This stud is mounted in 
the cross slide C, which operates on base D. Plate 
A is provided with two tapped steel bushings which 
are a forced fit in holes drilled and counterbored for 
the purpose. These bushings receive the two screws 
which secure the work in position on the fixture, 
with the purpose of preventing the rapid wear of the 
threads which would take place had they been tapped 
directly into the cast iron. The fixture is shown set 
in position for milling the parallel sides A, of the 
work. There are two tapered pins E and F, which 

Ch. 19 




Fig. 19-21. The milling fixture for the part shown in Fig. 19-20. Plate A is adjustable to three positions. 

are used for locating the work in the required posi- 
tion. For milling the parallel sides of the work, pin 
F is inserted in hole N to locate the cross slide C in 
the required position. Similarly, pin E is located in 
the central hole to locate swivel plated. These pins 
are merely used to locate the fixture; bolts G and // 
are provided to secure it in the required position. 
When the fixture is set for milling the angular side C 
of the work, pin E is inserted in hole /, and pin F in 
hole O. This sets swivel plated at the required angle 
and also locates the cross slide C at the required off- 
center distance to enable the work to be milled by 
the outer edge of the cutter. After this operation has 
been completed, swivel plate A is then swung over to 
enable pin E to enter hole K. Similarly, cross slide 
C is moved so that pin F will enter hole M. This 
brings the work into position to enable the angular 
side B to be milled by the outer edge of the other 
cutter on the arbor. 

The fixture described above is, in a sense, a 
primitive indexing fixture. Fully indexing fixtures 
are used extensively, and dividing heads of various 
types are available for the control of the indexing 

Movable milling fixtures are those that move 
while the cutter passes over the work. Radial fix- 
tures perform a slow rotation around a fixed center 
or axis, with the result that the cutter generates an 
arc of a circle. Other movable fixtures, similarly 

rotating around an axis, arc controlled in their mo- 
tion by a template and are, therefore, capable of 
generating curved surfaces of any desired shape. 

Locating Pins in Milling Fixtures 

Locating pins in milling fixtures can be fixed or 
retractable. An example of a fixture using fixed 
locating pins is shown in Fig. 19-22. The part is an 
aluminum cylinder head, and the fixture is rotating. 
The cylinder head is supported and located on the 
large diameter circular locator which centers it on 
the inside bore. The final accurate location for mil- 
ing the fins is obtained by means of a diamond- 
shaped pin which can be seen in the background. 
This pin engages the locating hole in the joining 
surface of the cylinder head. In this operation, the 
cooling fins are milled to the required depth by 
guiding the cutter with a tracer which follows the 
contour of the template as the piece rotates with 
the fixture. 

Retractable pins are used where a workpiece must 
slide into position before the pins can engage. An 
arrangement of retractable pins is shown in Fig, 
19-23, where the locating pins are mounted on the 
ends of a cross bar. This cross bar, balanced by two 
springs placed at equal distances from a centrally 
located eccentric, is moved up and down as the 
eccentric is operated by a hand lever. The lever is 



Ch, 19 

Courtesy of Cincinnati Milacran Inc. 
Fig. 19-22. A rotating fixture for milling cooling fins on a cylinder head with one fixed diamond pin for locating the part, 

located at the front of the machine (Fig, 19-24), der block held in a fixture (not shown in the 
which is used for milling various surfaces on a cylin- picture). 





Side stop 



Courtesy of Cincinnati Milscron Inc. 
Fig, 19-23. A milling fixture with retractable locating pins. 

Ch. 19 



Courtesy of Cincinnati Mikcron Inc. 

Fig. 1 9-24. The same milling fixture shown in position on (tie milling machine. 

The locating pins are normally in the retracted 
position. When the work is moved into position, 
the operator rotates the lever to raise the pins, and 
also moves the cylinder block slightly to ease the 
engagement of the pins with the locating holes. 
After the part has been located and clamped, the 
pins are again retracted. To further facilitate the 
insertion of the pins and thus reduce the time re- 
quired for locating the part, both locating pins are 
made with the diamond-shaped head and both are 
located with their major axis perpendicular to the 
surfaces to be milled. This will permit a slight 
variation in the location of the part in the direction 
parallel to the surface to be milled. The movement 
provides ease in positioning of the parts and has no 
appreciable effect on the accuracy of location of the 
cylinder block for the milling operation. 

Gear (-Jobbing Fixtures 

Successful gear nobbing depends equally on the 
accuracy of the gear blanks and the accuracy and 
rigidity of the hobbing fixture. A typical hobbing 
fixture (Fig, 19-25) for a vertical spindle hobbing 
machine consists of a base bolted to the machine 
table, a bottom support plate, a mandrel, an upper 
clamping plate, and a clamping nut. The mandrel 
extends above the nut with a pilot, which is sup- 
ported by the supporting arm of the machine. The 
following points are highly significant: The gear 

blanks are accurately centered on the mandrel; they 
are supported and clamped on the largest possible 
diameter, and before the base is finally clamped to 
the table, the entire fixture is centered, with respect 
to the axis of rotation, by a dial indicator. To 
allow for this centering adjustment, the base must 
not be solidly centered in the machine table; there 
must be about 1/8-inch (3-mm)-clearance in the hole 
in the machine table for the pilot of the base. The 
height of the base must be sufficient to allow for a 
clearance of about 1 inch from the work to the 
cutter at the lower end of the travel. Adequate 
approach and overtravel must be provided at both 
ends of the cutter travel, and the fixture designer is 
cautioned that overtravel, in the case of helical gears, 
is considerable yet not easily detected from a drawing. 

Fixtures for N/C Milling 

The rapidly expanding use of numerically control- 
led (N/C) milling machines has focused attention on 
the need for reduction of all phases of non-cutting 
time. The loading and unloading time is not a 
programmed operation, and even with skillfully 
designed fixtures, is still a burden on the economy 
of the operation. It can be drastically reduced, how- 
ever, by dual fixturing; that is, by the use of two 
identical and interchangeable fixtures, a method 
which is used quite extensively. While one part, 
clamped in its fixture, is machined, the otheT fixture 



Ch. 19 

Direction of hob rotation 
for convention*! 8nd 

fU-tl h.Gbrlr,;- T.ethbOd* of 


*■ ^ / 

Hob Slide 




I'ig. 19-25. Gear hobbing fixtures, a, 

Courtesy of Gould & Eberlmrdt Gear Machinery Corp. 
Plain, b. With reversible bottom plate. 

is unloaded then reloaded. When space permits, 
both fixtures are mounted on the machine table. 
With very large fixtures, it is necessary to unload 
and reload one fixture on the floor, while the part 

in the other fixture is machined, and then the fix- 
tures are exchanged. An example of this type of 
operation is seen in Fig. 19-26, 

Courtesy of Cintimatic Division of Cincinnati Milacron Inc. 
Fig. 19-26. The use of interchangeable fixture? with an N/C milling machine. 



Design Studies III — Miscellaneous Fixtures 

Lathe Fixtures in General 

Lathe fixtures are, for the most part, used on 
vertical and horizontal turret iathes and high-speed 
production lathes, In the past they have not been 
used on engine lathes to the extent they deserve; a 
skillfully designed yet inexpensive fixture can well 
convert an engine lathe into a production machine. 
However, fixtures are now being used extensively on 
engine lathes equipped with N/C controls, Work- 
pieces are centered, located, and clamped in lathe 
fixtures in essentially the same way as in conventional 
workholdtng devices used with lathes: accordingly, 
the fixtures can be classified as chucks with special 
jaws and inserts, collet-type fixtures, face-plate fix- 
tures, pot-type fixtures, mandrels and arbors, and 
special fixtures. 

Since cutting forces are unbalanced, lathe fixtures 
are always designed with large metal thicknesses and 
strong clamps. For the main fixture body, gray cast 
iron is preferred to mild steel for damping vibrations; 
however, for high-speed operation, weight and 
strength considerations may require the use of steel 
(in welded or built-up construction) rather than gray 
cast iron. With the exception of mandrels, which 
are supported on the tailstock, lathe fixtures are 
cantilevered. They must be designed with as little 
overhang as possible and, for operator safety, pro- 
jecting screws and pins must either be avoided or 
shielded. At medium and high spindle speeds, cen- 
trifugal forces become significant and must be taken 
into account. They affect clamping forces and will 
cause vibration if the fixture with the part does not 
run true. Many fixtures for second operation cuts, 
taken at high spindle speeds, are therefore designed 
so that they can be adjusted with respect to the 
spindle axis. Examples of adjustable fixtures are 
shown in Figs. 6-43 and 6-44. Irregularly shaped 
workpieces must be counterweighted. Medium- and 
large-size lathe fixtures for roughing work in a first 

operation are usually designed on the three-point 
principle. Larger fixtures for second operations and 
for thin-walled parts are designed with equalizing 
clamping devices (the floating principle). Examples 
of applications are shown in Figs. 9-11 and 12-5. 

Chuck Fixtures 

The cheapest type of lathe fixture is the standard 
lathe chuck (3- or 4-jaws), with special jaws or 
inserts, machined to fit the part. Aluminum, gray 
cast iron, and steel inserts are commercially available 
in dimensions to match standard chucks. As a 
general rule, steel jaws with serrated or hardened 
surfaces are used for gripping on rough parts, while 
soft jaws with smooth surfaces are used on machined 
parts to prevent scratching or marring These rules 
also apply to power operated chucks, found on many 
turret lathes and production lathes. Designs of more 
elaborate chuck fixtures are also shown in Figs. 
9-11 and 12-5. 

Since centrifugal force tends to draw the jaws away 
from the part (except when the part is clamped 
from the inside), and the standard chuck design is 
inadequate at spindle speeds used in high-speed pro- 
duction lathes, solutions must be sought; i.e., using 
a power operated chuck where a positive clamping 
force is constantly maintained on the jaws. Another 
solution is to make use of the centrifugal force for 
clamping. An example is shown in Fig, 20-1. The 
machine is a two-spindle production lathe and the 
part to be turned, in this case, is a cast-iron brake 
drum. The rear chuck is shown loaded; the front 
chuck, empty. The boss of the part is clamped by 
means of the hook clamp seen near the center of the 
chuck. In addition, the part is clamped on its 
periphery by 10 jaws. Each jaw can rotate around 
a fulcrum and at the back it is provided with an 
inertia block of greater mass than the forward 




Ch. 20 


Courtesy of Heald Machine Div., Cincinnati Milacron inc. 
Fig. 20-1. Lathe fixtures (chucks) where jaws are actuated 
by centrifugal force (inertia fingers). 

clamping end of the jaw. The result is that when 
the spindle rotates, the inertia block is forced out- 
ward by centrifugal force, and, in turn, forces the 
clamping end of the jaw against the work. Con- 
sequently, the clamping pressure increases with the 
spindle speed, Uniform pressure applied around the 
rim of the brake drum does not distort the part and 
eliminates chatter when the drum is machined. 

The principle of the collet chuck was shown in 
Fig. 9-3, with design rules and data presented in the 
accompanying text. The total travel of the jaws or 
fingers is determined by elastic deformation within 
the collet and is, therefore, very short. 

The maximum diameter range of the spring collet 
is 0.002 to 0.003 inch (0.05 to 0.08 mm) and it is 
intended for use on bar stock and bar-shaped parts. 
However, the term "collet chuck," as applied to 
fixtures in general, is now used in a wider sense. 
Any chuck that actuates its jaws by an axial motion 
relative to a conical surface is a collet-type chuck. 

The two chucks for the centering of gear wheels 
shown in Figs, 9-22 and 9-23 are collet chucks. They 
are grinding chucks, but that is a matter of applica- 
tion rather than of design. The collet principle can 
be applied to chucks of any dimension and propor- 
tion, and for internal as well as for external clamp- 
ing. A collet-type lathe chuck for interna! clamping 
is shown in Fig. 20-2. ' It is operated by means of the 
central draw bar. The actuating cone is a solid 
conical plug in the center of the chuck. The sliding 
pads are kept in contact with the cone by springs 
and each carries two pins which constitute the actual 

E. Thaulow, Maskinarbejde (Copenhagen; G.E.C. Gad's 
Forlag, 1930) vol. 11. 

Courtesy' of E. Tlmulow 
Fig. 20-2. A collet-type lathe chuck for internal clamping. 

jaws. As the cone is drawn in, the jaws are forced 
out. Dotted lines indicate buttons that can serve as 
axial end stops. 

The collet chuck can be combined with other 
locating and clamping components. One example is 
shown in Fig. 20-3. The part is a pinion, integral 
with a long shaft, too long to be held in any type of 
chuck. For the same reason, the collet chuck pro- 
vides excellent centering of the part and is well 
adapted to precision work. An application example 
is shown in Fig. 9-24. 

Face Plate Fixtures 

The face plate fixture is the natural type of fixture 
for machining large diameter parts on the vertical 
turret lathe. It is highly versatile and any type and 
combination of locators and clamps can be built up 
on it. A typical example is shown in Fig. 20-4. 
The work A is a cast-iron bracket which has 
previously been machined along the face D and 
has had the tongued portion cut approximately 
central with the cored hole at Y. Four holes have 
also been drilled at /. Two sizes of these brackets 
are made in lots of ten or twelve. An angle plate 
B, is tongued on the underside F, to fit one of the 
table T-slotsand is held down by screws (not shown). 
The distance E, for the two sizes of brackets, is 
determined by placing a stud G in the center hole 
of the table and locating angle plate B from it. The 
bracket is placed in position on the angle plate so 
that tongue H fits into the groove, and bolts / are 
passed through the holes in the bracket and tightened 
by nuts K. Clearance is provided in the bolt holes 

Ch. 20 



Courtesy of Heald Machine Div., Cincinnati MUacron Inc. 
[■'ig. 20-3. A collet-type lathe chuck for external clamping of a pinion with a long shaft (inverted tailstock). 

Fig. 20-4. A face plate fixture for noncttcular bracket-type part?. 



Ch. 20 

Courtesy of Monarch Machine Tool Co. 
Fig. 20-5. A lathe fixture for a pump body. a. Front and end views; b. Rear view. 

to allow the finished edge of the bracket to rest on 
pins C. Two special jaws Q are fixed in position on 
the table but may be adjusted radially, when nec- 
essary, to bring them into the correct position for 
the other size of bracket. The jaws are provided 
with set-screws O, which are adjusted to support the 
overhanging end of the bracket, after which they 
are locked by the check nuts at P. The jaws are 
keyed at 5 to the sub-jaws of the table; and clamps 
N are used on the unfinished portion of the bracket. 
The clamps are tightened by nuts at R, so that the 
surface to be machined is clear of interferences. The 
boring bar L is used to bore the hole, and the side 
head tool M faces the pad. This fixture illustrates 
that lathe fixtures can be made to accommodate 
workpieces of completely noncircular shapes and of 
more than one size. 

On engine lathes and horizontal turret lathes it is 
often more convenient to have a special face plate 
integral with the fixture and to mount the complete 
assembly on the spindle nose, as was shown in 
Figs. 6-43 and 6-44. A fixture for the facing and 
internal machining of an aircraft fuel-pump housing 
on a numerically controlled lathe is shown in Fig. 
20-5a and h. The fixture base is mounted on the 
face plate of the lathe. The part is located and 
clamped in V's, which are carried on a projecting 
plate and braced against the fixture base. 

The pot type fixture is used for parts of large 
diameter and considerable axial length, which do 

not require external machining. A typical example 
is shown in Fig. 20-6. The work A is a large casting 
which, because of its dimensions (diameter, length, 
and wall thickness), could not be adequately sup- 
ported and driven by a conventional chuck. The 
variations in dimensions which may be expected in 
large castings require a liberal clearance between the 
work and the fixture. In the present case, a clear- 
ance of one inch was provided. The pot fixture H is 
centered on the lathe tabic by the locator / and 
bolted down in the T-slots with bolts O. The part is 
located by its cylindrical surface against three screws 
B, B, and C, and is first positioned against these 
three points by the central clamping screw D. After 
the positioning is accomplished, the clamping is 
completed by application of the upper and lower 
screws D, and all three screws are then moderately 
and evenly tightened. In the vertical direction, the 
part is supported on three points F, F, and G (G is 
fixed, while F and F are adjustable). Windows/ 3 in 
the fixture wall allow for access to the adjustable 
supports. The part is clamped down by means of 
U-clamps L and nuts and washers M on studs K. 
This fixture presents several interesting details: the 
screwthreads F on clamping screws D and the adjust- 
able points F are shielded against chips; screws D 
are provided with "snubbers," heavy rubber pads on 
their tips which prevent distortion of the casting 
wall because of excessive pressure and also assist in 
the damping of vibration (chatter) during machining. 

Ch. 20 



SECTioti x-y-2 
Fig. 20-6. A typical pot type fixture. 

Mandrel and Arbor Type Fixtures 

Mandrel and arbor type fixtures will center, locate., 
and grip the work from the inside and are normally 
used for parts that already have a machined internal 
surface. The mandrel is supported at both ends as a 
simple beam; the arbor is carried at one end only, as 
a cantilever beam. In the simplest case, the work is 
centered with a sliding fit, located endwise against a 
shoulder, and clamped with a nut and washer. Com- 
mercially available mandrels and arbors hold the 
work between a fixed and a movable cone. Another 
type holds the work on an expanding sleeve. This 
design is shown in Fig. 20-7 } The sleeve is cylin- 
drical on the outside and fits inside on a taper on the 
mandrel body. When forced axially up onto the 
taper, the sleeve expands and clamps the part. The 




Courtesy of E. Thautow 
Fig. 20-7. A mandrel with an expanding split sleeve. 

sleeve is slit, with slots running alternately from 
either end. The large nut on the left-hand end of 
the mandrel serves the double purpose of locating 
the part endwise and of releasing the pressure by 
forcing the sleeve down on the taper. These man- 
drels are made with tapers varying, in general, from 
1:50 up to 1:15; and occasionally up to 1:6. A 
1:50 taper represents the limit for what conveniently 
can be operated with a release nut. This type of 
mandrel fixture is perhaps the most satisfactory of 
all. It locates and centers well and the sleeve re- 
mains practically cylindrical as it expands, exerting 
a uniform pressure on the part. Sleeves of different 
sizes can be used on the same mandrel body. For 
satisfactory results, the number of slots must be 
related to the sleeve diameter as follows: 


Number of Slots 



up to 2 
up to 3 
up to 5 
up to 7 

up to 50 
up to 75 
up to 125 
up to 175 

2X 3 

2X 5 

A recent development, now commercially avail- 
able, is a mandrel, or arbor, with one or several 
hydraulically expanded sleeves. An example is 
shown in Fig. 20-8, where two sleeves are used to 
clamp a part with a stepped hole. Hydraulic pressure 
is generated by means of a piston forced against the 
hydraulic fluid by an actuating screw, which is 
manually rotated with a socket-head screw wrench. 
For high-production work, the piston can be actuated 
by a push rod mounted centraOy in the lathe spindle. 
Mandrels and arbors can be dimensioned by the same 
rules as boring bars. Details concerning the design 
and dimensioning of the expanding sleeve and the 
data for the hydraulic system are presented in Chap- 
ter 21, Universal and Automatic Fixtures. 

Another recent development is the mandrel with 
an expanding sleeve made of an elastomer. It was 
developed by AEC-NASA and its description is pub- 
lished in a Tech Brief. 3 The purpose is to support 

E. Thaulow, Maskmarbe/de (Copenhagen: G.E.C. Gad\ 
Forlag, 1928) vol. I. 

3 An AEC-NASA Tech Brief from Cutting Tool Engineering, 
April 1969, p. 17. 



Ch. 20 


Courtesy of Hydra-Lock Corp. 
Fig, 20-8, An arbor with two hydraulic ally expanded clamping sleeves. 

rough, hollow castings during grinding and turning. 
The device is shown in Fig. 20-9. The elastomer 
sleeve is supported on a mandrel threaded at one 
end. The part is slipped over the sleeve, a heavy 
washer and nut are put in place, and when the nut 
is tightened, the sleeve expands on its diameter 



Fig. 20-9. A mandrel with an expanding elastomer sleeve. 

while maintaining constant volume, and centers, 
supports, and clamps the part. The elastomer must 
be nearly completely enclosed so that il cannot 
escape from the pressure. Under these conditions it 
behaves much like a rigid body after it has filled the 
cavity inside the part. It locates and centers the 
part on its average inside diameter or contour. The 
elastomer can be cast in aluminum molds. The 
diameter of the sleeve is made 0.010 to 0.015 inch 
(0.25 to 0.38 mm) smaller than the cast hole and 
the length approxiamtely 1/8 inch (3 mm) larger. 
Elastomers suitable for this application are silicone 
RTV, PVC (polyvinyl chloride) plastisols, vulcan- 
ized rubber, and polyurethane. Generally, elastomers 
can be Teused. 

Courtesy ofE, Thaulow 
Fig, 20-10. A simple type of arbor for holding threaded 

Workpieces with unmachined interior surfaces, 
such as cored holes, can be mounted on mandrels or 
arbors with movable internal jaws. 

Parts with an internal thread can be mounted on a 
mandrel. The simplest case is that shown in Fig. 
20-10. The screw thread serves to center, clamp, 
and drive the part, and alignment is provided by the 
flat shoulder. A plain arrangement such as this 

Courtesy ofE. Thauiow 
Fig. 20-1 1. An improved type of arbor for threaded work, 
with a separate clamping and release nut. 

E. Thaulow, Mttskinarbejde (Copenhagen: G.E.C, Gad's 
Forlag, 1 928) vol, I. 

Ch. 20 



causes the part lo bind in the thread after machining. 
An improvement, shown in Fig. 20-1 1, uses a large 
clamping nut with a coarse left-hand screw thread. 
The nut provides the means for end-locating the 
shoulder and for aligning the part. After machining, 
the nut is backed off a fraction of a turn and the 
binding pressure on the part's screw thread is 

Miscellaneous Fixtures 

Lathe operations on parts that are unusual because 
of their shape or dimensions offer many opportuni- 
ties for the successful application of fixtures. At 
times the fixtures are complicated and expensive, at 
other times they are simple, if not primitive, yet 
they are always efficient with respect to the saving 
of time and the improvement of quality. A system- 
atic classification will not be attempted here but a 
few examples will indicate the possibilities. 

A crankshaft has one geometrical axis defined by 
the main bearing journals, and one or more addition- 
al axes, each defined by a wrist pin or a set of wrist 
pins. Each of these axes requires a turning operation, 
but only the axis through the main journals termin- 
ates in solid steel with surfaces that can be center 
drilled. The other axes primarily run through air. 
Two different sets of fixtures for the turning of 
crankshafts are shown in Fig. 20-12, a and b. In 
example a, each end of the crankshaft is provided 
with a block that carries the two sets of centers 
required for the two wrist pins. The center block at 
the tailstock end can also carry a counterweight, as 
indicated by the dotted lines. Other fixture com- 
ponents needed are struts foT taking the axial thrust 
between the tailstock and the spindle center, and 
spacers between two parallel arms. The fixtures 
shown in example b are comprised of two brackets; 
the one to the left is clamped to the spindle, and 
the one to the right is provided with a center drilled 
plug that is supported by the center in the tailstock. 
The fixture shown in Fig. 20-13 carries, supports, 
and guides the free end of a tank for a space explor- 
ation rocket engine. The visible portion is the so- 
called "Y-joint," which is seen being machined in 
preparation for welding to the end closure and the 
adjacent tank. The fixture is provided with adjust- 
able bearing blocks all the way around to allow the 
tank to rotate accurately, just as an axle rotates in 
an ordinary steady rest. 


5 Ibid., vol. 11. 

Courtesy of t\ Tkautow 
Fig. 20-12. Fixtures for turning crankshafts. 

Boring Fixtures 

Boring is an operation whereby an existing hole is 
machined to a larger size. Boring fixtures differ 
from drill jigs in that they are to be used with 
boring bars. Drill jigs, however, can also be de- 
signed for combined drilling and boring operations. 
While the twist drill is supported and guided by the 
hole that is being drilled, the cutter in a boring bar 
receives its support entirely from the boring bar it- 
self, producing holes of greater accuracy with respect 
to diameter, roundness, position, and alignment. 

Boring bars differ in design and can be made in a 
rather wide range of sizes. Some boring bars, called 
"line" boring bars, are supported at both ends. 
Others, called "stub" boring bars, are supported 
only at one end by the spindle of the machine. Line 
boring bars are used to bore long, deep holes and 
holes of very large diameter. Stub boring bars are 
used more frequently than line boring bars and must 
be used for boring blind holes. 

Boring operations are performed on many types of 
conventional or numerically controlled machine 
tools. Conventional or N/C lathes, milling machines, 
drilling machines, vertical boring mills or vertical 
turret lathes, and horizontal boring mills (HBM) are 
used extensively to perform boring operations. The 
horizontal boring mill is a remarkable machine, 
practically a one-man machine shop. With different 
accessories, this machine can perform almost any 



Ch. 20 

Courtesy of Aerojet-Genera! Corp. 
Mg. 20-13. "Missile Maker Lathe," for machining laigc missile components. 

conventional machining operation. Its spindle as- 
sembly is designed with the precision and rigidity 
required for work with boring bars, and it has the 
necessary power for the boring of large holes. By 
its design, and with experienced and careful opera- 
tion, it produces holes that are exactly parallel to 
the surface of its table. Boring, in the toolroom, is 
performed on jig borers to obtain very precise hole 
locations on tools, dies, jigs, and fixtures. Jig borers 
sometimes are used to obtain precise hole location 
tolerances on machine parts where the number of 
parts is relatively small. Boring fixtures occasionally 
are used on jig borers to speed-up the location of the 
part on the machine or where the part cannot be 
conveniently held by any other means. 

High-production boring is often done on special 
machine tools. Some are special purpose machines 
such as those found in the automotive industry. 
Often the part is automatically moved to and from 
the boring station by machines called "transfer 
machines." A different class of boring machine is 
comprised of the highly automated, high-speed pro- 
duction boring machines, known as "Bore- Ma tics,"® 
or equivalent trade names, which operate at high 
cutting speeds and fine feeds (borizing). These 
machine tools may be adapted to do a variety of 

jobs; however, they are usually set up to do a 

particular job for a prolonged period of time. 

Boring fixtures are always used with these pro- 
duction machines. 

Design of Boring Fixtures 

Case 14. Design a boring fixture for a box-shaped 
part with a bearing hole at each end. 

The part is schematically represented by the rec- 
tangular outline A in Fig, 20-14. The distance 
between the holes requires a line boring bar; con- 
sequently, this type of fixture is called a "line 
boring fixture." The usual five design steps apply 
here again, with some modifications and simplifi- 
cations, that are characteristic for most boring fix- 

1 . Usually, a part to be bored is already machined 
on one or several major flat surfaces. Here, the base 
surface of the part is machined, and is now located 
and supported on a matching flat surface on the jig 
base. In the side direction it is located by means of 
pins, stops, or with keys or dowel pins, if such 
components are provided for in the part, for use in 
the final assembly. Endwise, it is located to provide 
sufficient clearance at each end (dimensions B). 

Ch. 20 



2. The part may be clamped by means of clamp 
straps, but often the part is already provided with 
bolt holes which can then be used for clamping it to 
the fixture. 

3. Intermediate supports are usually not needed, 
since the part is well supported on its base. 

4. The cutter guides serve as bushings for the 
boring bar. The fixture is provided with fixed 
bushings mounted in brackets K, and the boring bar 
has slip bushings of sufficient outer diameter to 
allow the bar with its cutting tools to be inserted 
and withdrawn endwise. 

5. The fixture body consists essentially of a base 
with brackets K, for bushings. If needed, the 
brackets may be provided with stiffening ribs C. 
Side lugs D are machined exactly parallel to the 
boring axis and serve for the fixture alignment on 
the machine table, Bosses E arc for measuring and 
checking the clearance B, to the end surfaces G, of 
the work. B must be large enough to allow for 
variations in the size of the raw part, for the escape 
of chips, and, if necessary, for the insertion of facing 
cutters or the mounting of shell reamers on the bar. 
The fixture is bolted to the machine table by means 
of three lugs //. The use of three lugs provides for 
statically determinate support and minimizes the 
possibility of elastic deformation (springing) in the 
fixture. To reduce weight, the fixture base is cored 
out from below, and the bushing bosses are tapered. 
The fixture is a one-piece casting. Gray cast iron is 
preferred over weldments because of its excellent 
damping properties. 


n F ± 

L -k 

l'ig. 20-14. A line boring fixture cast in one piece. 

Many modifications are possible from this general 
outline. The fixture can be built with a base plate 
and detachable brackets carrying the bushings. This 
is used when the dimensions are so large that the use 
of a single casting is impractical. It also provides 

for lengthwise adjustment of the brackets to accom- 
modate workpieces of different lengths, for the inser- 
tion of an intermediate bracket in the case of very 
long workpieces, or for the use of different brackets 
on -the same fixture base. For multiple boring 
operations the fixture can be provided with a multi- 
ple spindle gearbox which rotates and feeds the 
boring bars. It should be remembered that mating 
gear wheels rotate in opposite directions, and some 
cutters may have to be designed for left-hand cutting. 
A boring fixture can have bushings for holes in more 
than one direction and is then placed on a revolving 
or indexing table. 

A somewhat different technique can be used when 
the boring operations affect only a small area of a 
much larger machine part or assembly. In that case, 
the boring fixtures are small and are carried by the 
larger unit. This method is frequently used in the 
manufacture of machine tools. One example is 
given in Fig, 20-1 5, which shows a machine tool 
bed with some accessories. The hole B which signi- 
fies the hole for the main bearing in the headstock E, 
is shown bored with a boring bar supported in two 
fixtures C and D which are located on the inverted 
V's of the machine bed. This ensures alignment 

Fig. 20-15. Example illustrating the use of the workpiece as 
a guide for the boring bar. 

between bed and spindle. Next it is assumed that 
hole B should be aligned with holes F and G in two 
already existing carriages or brackets, and in this 
case these same holes serve as guides for the boring 
bar, if necessary by the use of liner bushings. 
Finally, the front elevation shows how hole ./ is 
boTed in the carriage and apron / by using the three 
bearings K, L, and Af as guides for the boring bar. 
A tapered hole is bored by means of a boring bar 
mounted at the required angle in a bushing in such a 
way that it is fed through the bushing, rotating the 
bushing by means of a key and key seat. The 



Ch. 20 

Courtesy of E. Thaulow 
Fig. 20-16. A boring bar with bushing for boring a tapered 

The workpiece is a casting for a lathe headstock, is 
located on inverted V's / and 2, and clamps against 
end stop 4 by means of screw 3. When located, it is 
finally clamped down by screws 5. The fixture has 
boring bar bushings 6 for the spindle, 7 for the back 
gear shaft, and 8 for the rocker shaft. The fixture 
body is designed like a box-type drill jig, and all 
operations are performed in a radial drill. 

Design of Boring Bars 

The strength and rigidity of a boring bar is de- 
termined by its diameter D and free length /.. The 
diameter should always be taken as large as possible, 
allowing a chip clearance between the bar and the 
raw hole of not less than the machining allowance 
in the hole. The free length L can be taken as 
for line boring bars 

L < IQD 

for stub boring bars 

L <6D 

The length of a bushing should never be less than/), 
and for smaller bars, the bushing length may be 
taken up to 2 time's D. This rule is for boring bars 

Courtesy of E, Thaulow 

Fig. 20-17. The boring fixture for the operation shown in Fig. 20-16. 

arrangement is shown in Fig, 20-1 6. 6 The boring 
bar is driven through a universal joint. This detail 
is part of the boring fixture shown in Fig. 20-17.* 

E. Thaulow, Maskinarbejde (Copenhagen: G.E.C. Gad's 
Forlag, 1930) vol. II, 

to be made of steel. Solid cemented carbide bor- 
ing bars may be dimensioned by comparison with 
previously used boring bars. The critical property 
is the transverse rupture strength which can vary 
greatly (up to 200 to 300 percent) in any given 
carbide grade. 

Ch. 20 



Fixtures for Production Boring Machines 

High-speed production boring machines use stub 
boring bars and other cutting tools (fox facing, 
countcrboring, etc., and sometimes for external turn- 
ing) that do not require bushings in the fixture. 
The fixtures are designed for quick precision clamp- 
ing of the part. As an example, Fig. 20-18 shows 
the fixture for machining a die-cast-aluminum engine 
front cover. With four spindles, the machine bores 
and faces pockets and shaft bores to a finish of 120 
to 125 RMS at a rate of 57 pieces per hour. The 
part is positioned from its rear side and is clamped 
by eight clamps, all air-operated for fast opening 
and closing. Shown in Fig. 20-19 is an indexing 
fixture that holds two compressor crankcases for 
simultaneous machining (finish boring and chamfer- 
ing of both ends of the cylinder bores). 

Grinding Fixtures 

The grinding operation is characterized by small 
cutting forces, high accuracy, and, in general, a large 
flow of coolant. Grinding fixtures must allow for 
the unrestricted access of coolant to the work, as 
well as free drainage of the used coolant, with no 
sumps or pockets where sludge can accumulate. 
Magnetic face plates and chucks, supplemented by 
electrostatic and vacuum operated devices for non- 
magnetic workpieces, aTe widely used as standard 
work-holding devices for surface grinding, but are 

Courtesy ofHeald Machine Div., Cincinnati Miiacron Inc. 
Fig. 20-19. An indexing boring fixture for two parts. 

not usually part of the fixture. The magnetic face 
plate can be used, however, as a fixture base that 
offers a fast and convenient method of removing 
and replacing the fixture. Typical examples of grind- 
ing fixtures of the chuck type were shown in 
Chapter 9, Figs. 9-22 through 9-24. Clamping is 
mainly done with hand or finger operated mechani- 
cal elements, designed for light duty only, and is 
not apt to cause distortion of the part, nor restrain 
its natural thermal expansion. 

The structural design of grinding fixtures is very 
similar to that of other fixtures, except that they 
are lighter. A fixture that structurally lends itself 

- ■■ -.-fgtfB&MI 


^^1 wM* 

"t ■* UttF WW, '> 

*JrM*fy s !ni 1 I 

Courtesy of Heald Machine Div.. Cincinnati Miiacron Inc. 
Fig, 20-18. A boring fixture with air-operated damps for high production. 



Ch. 20 

Courtesy of LeBlond Inc. 
Fig. 20-20. A jig grinding fixture designed as a 90 degree, 
til table angle plate. 

change the setup to the position for the second set 
of bores, the angle plate is placed on its back, there- 
by tilting it 90 degrees. 

Angle Plate Fixtures 

The internal grinder with planetary spindle motion 
is used for internal grinding operations on cylinder 
blocks and other similar type work. A fixture for 
this kind of operation is shown in Fig. 20-2 1. 7 
The grinder spindle is horizontal and the fixture is 
formed as an angle plate with stiffening end walls. 
The cylinder block is clamped by its base surface 
on the angle plate which has openings for the 
entrance of the grinder spindle. This fixture also 
has two diamonds for the truing of the grinding 
wheel, a feature that is characteristic of "many grind- 
ing fixtures. There are two reasons why a grinding 
fixture cannot use cutter guides of the usual type 
for setting the grinding wheel; one is that the set- 
ting would soon be lost because of wheel wear; 
the other is that an ordinary cutter guide would 
soon be ground down and destroyed by accidental 
contact with the grinding wheel. Therefore, instead 
of cutter guides, grinding fixtures are equipped with 
diamonds for the truing of the grinding wheel. The 
diamonds are preset for the final work dimension 
and the grinding wheel is run back past the diamond 
and trued before it starts the finishing cut. The 
angle plate is also a characteristic feature of many 
grinding fixtures. An angle plate has sufficient 

Courtesy ofE. Thaulow 

Fig. 20-21 . An angle plate fixture for internal grinding. 

to a milling fixture is shown in Fig. 20-20. The 
part requires the internal grinding of bores with two 
perpendicular axis directions. The machine is a jig 
grinder, and the spindle is positioned from hole to 
hole by means of the positioning mechanism of the 
machine table, in the same manner as on a jig borer. 
The fixture is an angle plate with a sloping front. To 

rigidity to withstand the weak grinding pressures, 
and is frequently used to raise the work to a con- 
venient position above the grinder table. 

E. Thaulow, Maskinarbejde (Copenhagen: 
1'orlag, 1930) vol. II. 

G.E.C. Gad's 

Ch. 20 



Automotive Grinding Fixtures 

Of the grinding fixtures used in the automotive 
industry, two are particularly significant because of 
their special design features which may well be 
utilized in other applications. They are the fixtures 
for the cylindrical grinding of crankpins and for the 
contour grinding of camshafts. 

The principle of a crankpin grinding fixture is 
shown schematically, and somewhat simplified, in 
Fig. 20-22. The work spindle of the grinder has a 
large face plate A, which carries a locator B, C for 
the main bearing journal at the end of the crank- 
shaft, and an index plate D with an indexing mech- 
anism, here shown simplified as an index pin E. 

Fig. 20-22. Schematic view of a fixture for the grinding of 
four crankpins. 

The locator consists of a 1/3 bearing shell B, and 
a movable pressure foot C. A similar locator is 
provided at the opposite end of the crankshaft. 
The locators are mounted in such a way that the 
crankshaft center One is offset from the woik spindle 
center line a distance equal to the radius to the 
crankpin centers. This distance is adjustable so that 
it can be changed, when needed, to accommodate 
other crankshafts. In the case shown, the adjust- 
ment is done by an exchange of bearing B. The 

crankshaft is nested in bearings B, and one crank- 
pin is brought into contact with a retractable loca- 
tor F, which brings one crankpin center line to 
coincide with the work spindle center line. These 
operations define the starting position for the crank- 
shaft and when that position is reached the pressure 
feet C close; the end of the crankshaft is clamped to 
the index plate /J; and the crankpin locator F re- 
tracts. The grinding wheel G advances and grinds 
the crankpin. After grinding is completed, the index 
pin E is withdrawn, the index plate with the crank- 
shaft is indexed to the next position, the grinding 
wheel is aligned with the next crankpin, and grind- 
ing can continue. 

t-'ig. 20-23. A crankpin grinding fixture with work 

On most crankpin grinders available, the fixture 
is integral with the machine. The operations: 
loading and positioning the crankshaft, clamping, 
indexing, start and stop, feeding, retracting, and 
repositioning the grinding wheel, are all mechani- 
cally controlled. An example of such a mechanized 
and automated crankpin grinder is shown in Fig. 

Cam grinding is a copying operation. The cam 
contours are copied from a set of master cams; con- 
tact between a master cam and the master roller 
is maintained mechanically by means of a spring 
or a hydraulic cylinder with piston. The principle 
of cam grinding is shown schematically in Fig. 20- 
24. A common base A, attached to the bed of the 
grinder, carries fixed bearings B and C for, respec- 
tively, the shaft D for the cradle E and the shaft 
for the master roller F. The cradle, which is a cast- 
ing of substantial dimensions, carries bearings G 
for the work spindle on which are mounted the 



Ch. 20 

Fig, 20-24. Schematic view of a fixture for cam grinding. 

master cam set H and a chuck /, with a live center 
for locating, supporting, and clamping one end of 
the camshaft K. The opposite end of the camshaft 
is carried by a tailstock, also known as the "foot- 
stock." The free part of the camshaft is supported 
against the grinding pressure by a "work rest." 
As the work spindle with the master cam and the 
camshaft rotates, the cradle is oscillated around its 
shaft D, and with the grinding wheel L in operating 
position, the master cam contour is transferred to 
the cam that is being ground. The master roller is 
moved lengthwise from one disc to the other, on 
the master cam, when the grinding wheel is moved 
into position for the grinding of the next cam on 
the camshaft. A camshaft grinding operation is 
seen in Fig. 20-25. The illustration shows the 
cradle with its inverted- V guide, the footstock, the 
work rest on the middle of the camshaft, and the 
driving chuck. The master cam mechanism is covered 
within the housing at the left in the photograph. 

Fig. 20-25. A camshaft grinding fixture. 

Fixtures for Planing and Related Operations 

Planing, shaping, slotting, and broaching have 
many features in common which are reflected in 
the design of their fixtures. They are straight-line 
operations which, with the exception of broaching, 
are performed by a single-point tool and a reciprocat- 
ing motion. In planing, the work moves, while in 
the other operations, the cutter moves. In planing 
and shaping, the motion is horizontal; in slotting, 
the motion is vertical. Broaching is done, horizon- 
tally (pull broaching) and vertically (surface broach- 

Because of inertia forces at stroke reversal, the 
cutting speeds are rather moderate. Vibration is 
not a problem, but fixtures must be designed to 
withstand the impact from the tool each time it 
enters the work. Structurally, these fixtures are 
related to milling fixtures. They require substantial 
metal dimensions, positive and strong end stops to 
withstand the impact, solid bolting down upon the 
machine table, and strong clamps. The recipro- 
cating operations require ample end clearance rela- 
tive to the cut surface at each end of the stroke. The 
run-out distance at the exit end is relatively small 
(about 1/2 inch [13 mm] for shaping, and 1 to 2 
inches [25 to 50 mm] for planing) to allow the 
tool to clear the work and be lifted before the start 
of the return stroke, and at least 1 inch (25 mm) 
for slotting (vertical) to allow for the accumulation 
of chips. The run-in distance at the entry end must 
be long enough to allow for deceleration, stroke 
reversal, rc-acceleration, and some time for the side- 
wise feed motion to be completed. For planers 
this will require several inches, depending on the 
size, type, and condition of the machine, but for 
the other machines, 1 to 2 inches (25 to 50 mm) 
will suffice. Tool setting blocks are often used. 
For the machining (planing and shaping) of compo- 
site shapes, such as a dovetail or the inverted V's on 
a lathe bed, the setting block is formed as a template 
of the contour. (See Fig. 13-6.) 

Planing Fixtures 

While planing is the natural operation for long 
parts, it can also be economically applied to smaller 
parts when they are clamped in a gang fixture. An 
example is shown in Fig. 20-26. Inexpensive shap- 
ing fixtures can be made by the installation of 
special inserts on the jaws of the standard .machine 
vise. In the vertical machining operations (slotting, 
surface broaching) the cutter moves in a vertical 
path (see Fig, 20-27) and the main cutting force F^, 

Ch, 20 



Kig. 20-26. A gang planing fixture for twenty-three steel forgings. 

is directed downwards. Since the slotting machine 
table is horizontal, this cutting force assists in 
stabilizing the work. However, the thrust force 
Ff is horizontal and quite significant. It acts 
with its full value right from the beginning of 
the cut and generates an overturning moment on 
the work. It is therefore essential that slotting 
fixtures for tall workpieces are designed with a 
wide base. Similar considerations apply to fixtures 
for surface broaching. The thrust force Fj on 
the average, is equal to 1/2 of Fq and may in- 

Fig. 20-27. A slotting fixture. 

crease to the same value as Fq as the cutting 
edges become dulled by wear. 

Broaching Fixtures 

Broaching is characterized by a short machining 
cycle and, for economical production, also requires 
a short loading and unloading time. Broaching fix- 
tures for production work are, therefore, almost 
exclusively provided with automatic clamping de- 
vices. They can take many forms, even for almost 
identical workpieces. As an example of the variety 
in the design of broaching fixtures, Fig, 20-28 a and 
b show two different designs of a fixture for the 
broaching of the large end of an automotive connect- 
ing rod. In each design, the closing and clamping 
motion of the clamping arm is controlled by a con- 
toured slot in the arm, while, in a, the clamp is 
double and is actuated by a crank arm on a rotating 
shaft. In b, there arc two individual clamps, each 
one actuated by a power cylinder. 

The indexing principle is also frequently used for 
the purpose of reducing loading and unloading time. 
An application is shown in Fig. 20-29. The machine 
is not a duplex, but carries two identical broaches 
on the ram and machines two parts with each stroke. 
The parts are manually loaded into nests on the 
periphery of the indexing plate and are indexed into 
the machining station where they are hydraulically 
clamped. After machining, they are indexed to the 



Ch. 20 

Courtesy of Cintimatic Division of Cincinnati Milacron Inc. 
Fig. 20-28 a and b. Two different designs of broaching fixtures for an automotive connecting rod. 

Courtesy of Cincinnati Milacron Inc. 
Fig. 20-29. An indexing broaching fixture with two parts 
in eaeh station. 

unloading station where they are gravity unloaded 
onto a ramp. 

Fixtures for Internal Broaching 

A fixture for internal broaching consists essentially 
of a work support, known as an adapter, that trans- 
mits the main cutting force to the (vertical or 

horizontal) machine table, and an internal part (a 
plug) to guide the broach relative to the work and to 
provide support against the thrust force. A thin- 
walled or otherwise flexible workpiece requires a 
substantial work support (Figs. 20-30 and 20-31). 
A broach with an asymmetrical cut, such as the 
keyseat broach, is supported by a plug, and on a 
horizontal machine the plug has an extension (a 
horn, see Fig, 20-32) to prevent sagging of the free 
end of the broach. The thrust force and its reaction 
are now internal forces within the part. By means 
of parallel or tapered inserts, the keyseat depth can 
be varied, or a plug can accommodate broaches of 
different heights. For tapered cuts, the face of the 
work support and the bottom of the slot in the plug 
are machined to the appropriate angle so that the 
work is tilted with respect to the broach. 

Fig. 20-30. A broaching fixture for pull broaching. 

Ch. 20 



Fig. 20-31. A broaching fixture for a thin-walled part. 

Fig, 20-32. A broaching fixture with a hom for keyseat 

Survey of Assembly Fixtures 

Assembly operations are performed with the use 
of fasteners (screws, rivets, pins, stakes, stables, etc.) 
by permanent deformation of parts (crimping, swag- 
ing, bending of tabs, etc.), and by bonding or join- 
ing primarily through a thermal process (welding, 
brazing, or soldering). Most of the nonthermal 
assembly operations Lire used in the mass production 
of small- and medium-size articles and require highly 
specialized machinery where the fixture is closely 
associated or integral with the machine. One im- 
portant exception is the very large assembly fixtures 
used in the aircraft industry where the components 
for wings, fuselages, spars, or control surfaces, are 
positioned find held while they are riveted or screwed 
together. In some cases, drilling is done in the 
same fixture, prior to the actual assembly. 

Aircraft Tooling 

No other industry exceeds the aircraft and aero- 
space industries in the use of fixtures of large 

dimensions and in the integration of different fix- 
tures into one master fixture system. The following 
description of a typical aircraft fixture system 
refers to the tooling for the aft wing section of a 
delta wing of a medium-size bomber. The section is 
612 inches wide, 261 inches long, and 21 inches 
deep in the landing gear wheel well area. Each 
assembly consists of skin panels and spars, bulk- 
heads, and box sections which form the under- 
structure. The assembly working area in the plant 
is occupied by a large steel structure, shown in Fig. 
20-33, which supports the individual fixtures, pro- 
vides work platforms in three levels, and contains 
services for electricity and compressed air, with 
overhead tracks for electric hoists. For each of the 
wing sections, a vertical assembly fixture carrying 
pin locators, stops, and clamps is mounted on the 
main structure. Stops and locators, including loca- 
tors for the fuel lines, are installed by means of tool- 
ing gages, which simulate the items to be installed. 
With the tooling gages in position, skin panel tooling 
samples are located and fabricated to minimum gap 
clearances at plane intersection joints. Hole loca- 
tions in the tooling samples are checked against the 
tooling gages and corrected as necessary. The setup 
for a typical operation is shown in Fig. 20-34. 
By means of the locators in the fixture, the under- 
structure is assembled, checked with gages, and 

The prcdrilled skin panels are located to the 

bulkheads and spars. Holes are transferred to the 
understructure and step drilled to size. Attachment 
holes in the skin are counterbored and countersunk, 
as required, attachment holes in the understructure 
are tapped, skin panels are removed, and the under- 
structure is de burred and cleaned. Master tooling 
gages simulating the main landing gear are used to 
establish the hole patterns for attaching the landing 
gear to the integral fittings of the corresponding 
bulkheads. Master tooling gages simulating the 
fittings for the elevon hinge, are used to control the 
installation of the locators for these fittings. 

Design of Are Welding Fixtures 

The type of assembly tooling most widely used 
throughout the metalworking industry is tooling for 
arc welding. In welding shop terminology they are 
called jigs when they are stationary, and fixtures 
when they are movable. Their purpose is to locate 
and hold the parts in correct relative position for 
joining, to reduce distortion, and to orient the part 
so that each weld can be laid in the most convenient 
position: i.e., downhand and horizontal. For this 
purpose, the fixture usually is carried by a positioner 



Ch. 20 

Courtesy of General Dynamics, Fort Worth Div. 
Fig. 20-33. Master assembly fixture structure in an aircraft plant. 

with which the part can be raised, lowered, tilted, 
and rotated. Positioners can be commercially avail- 
able machines, usually operated with a worm wheel 
drive to permit a wide range of positions, however, 
they operate rather slowly. An example is shown in 
Fig, 20-35. The fixture can be designed to in- 
corporate its own positioner, preferably designed 
as an indexing positioner, with which each operating 
position is secured by a locking device that enters 
a hole or a notch in the index plate. The system's 
center of gravity should be placed in the axis of 
rotation, if necessary by the addition of counter- 
weights. The balancing of the fixture is facilitated 
by the use of light alloys for the moving parts. Even 
large fixtures can be so well balanced that they can 
be operated manually and with one hand. Position- 
ing is fast, and operating positions are accurately de- 
fined and safely maintained. 

Distortion is caused by thermal expansion of parts 
during welding, and by subsequent shrinkage of 
deposited %veld material. Plain thermal distortion is 

transient; it disappears as the material cools down, 
and is harmless. To allow for thermal distortion, the 
parts may be firmly clamped at one place (an- 
chored), and allowed to slide against the friction 
under the clamps in directions away from the anchor 
point. Shrinkage distortion is of a different nature. 
Since the weld material is deposited from one side 
at a time, the initial shrinkage is essentially asym- 
metrical and tends to misalign the parts. It is 
calculated that the distortion is 1 degree per pass. 
Minimum lateral shrinkage is obtained by welding 
with large electrodes in as few passes as possible. 
The distortion is counteracted by the use of clamps, 
fixed locators, and stops in such places that they 
prevent or substantially minimize the anticipated 
distortion, and also by dimensioning the fixture 
body with adequate rigidity and strength. The 
active stresses to be encountered equal the yield 
stress of the welded material at the elevated tem- 
perature that prevails at the beginning of the cooling 
period after completed solidification. 

Ch. 20 



Filing Matter Gage 

Courtesy of General Dynamics, Fort Worth Div. 
Fig. 20-34. Locating and fitting of skin panel tooling samples for an aft wing section. 

A fixture is designed around or inside the complet- 
ed workpiece, and must allow the finished piece to 
"get out" again. For this purpose, internal fixtures 
may be collapsible and external fixtures may be 

split or have a large hinged or otherwise detachable 
door. Rams, bumpers, or other types of ejectors 
may be added for the removal of binding work- 

Courtesy of Aronson Machine Co. 
Fig. 20-3S. An are-welding fixture mounted on a boom type positioner. 



Ch. 20 

The parts to be welded are fitted with a space or 
gap of from 1/32 to 1/16 inch (0.8 to 1.6 mm) 
between edges. Backing bars (of copper, aluminum, 
or stainless steel) are placed behind the gaps to act 
as a heat sink to protect against overheating, burn- 
ing, or buckling, and to prevent blowout of molten 
weld material. Commerical copper suffices in most 
cases. Where severe wear is anticipated, Class II 
copper is preferred. This is a copper alloy with a 
chromium content, widely used for resistance weld- 
ing electrodes. Stainless steel is used where pre- 
heating or postheating is required as it resists 
oxidation. To prevent contamination, some exotic 
materials, notably titanium, are not permitted to 
contact other materials. In such cases the backing 
bar is provided with a groove, behind and somewhat 
wider than the gap between the plates. The groove 

A different technique is the use of subassembly 
fixtures, followed by the main assembly fixture. 
During subassembly welding, the parts are allowed to 
distort as needed, but the pieces are cut with over- 
length, and free ends are trimmed back and fitted 
together when installed in the main fixture. Figure 
20-36 shows a fixture of this category, used for the 
final welding of subassemblies for the engine frame 
of a Titan missile. 

The flow of electric current must be controlled. 
The fix tu re or the parts must be grounded to provide 
a return path for the current. Besides, the current 
carries a magnetic field, here called the "magnetic 
flux," It can affect the direction of the arc and 
disturb the welding (arc blow). The flux in the part 
and in the fixture is controlled by the path of the 
return current, and it is essential that the magnetic 

Courtesy of Aerojet-General Corp. 
Fig. 20-36. Main assembly welding fixture for the engine frame of a Titan missile. 

is purged with an inert gas under sufficient pressure 
to carry the penetration, which then forms a bead 
without contacting the backing bar. For large 
parts, two fixtures may be used. The first fixture 
is the "tacking" fixture and is for tack welding only; 
the second fixture, known as the "holding" fixture, 
is for the completion of the welding. 

flux has no opportunity to cross or concentrate 
near the path of the arc. A rule-of-thumb recom- 
mends that in the vicinity of the joint, steel members 
should be one inch (25 mm) below the part or two 
inches (50 mm) above the path of the arc. An 
effective means is to ground the part at the starting 
end of a long weld. If clamps and fixture are made 

Ch. 2C 



of nonmagnetic material, which will not provide a 
path for the magnetic flux, the magnetic field will 
be weak. 

Welding fixtures are of simple and inexpensive 
design with liberal tolerances and little or no ma- 
chining. As a general rule, no castings are used, but 
all the standard structural shapes, plates, angles, 
channels, and I-beams, can be employed. Preference 
is given to closed sections, such as circular, rectangu- 
lar, and square tubes, because they combine high 
torsional strength and rigidity with low weight. 
Toggle clamps are used extensively; they are in- 
expensive and allow fast operation. 

The screw threads on clamping bolts must be 
shielded against weld splatter. 

Welding fixtures within some size limitations can 
be made from castable epoxy and phenolic tooting 
resins. Since they are cast to shape, they frequently 
permit the work to be located in the fixture by 
simple nesting without the need of additional loca- 
tors and clamps. The plastic materials are light, 
have satisfactory dimensional stability, minimum 
deterioration, and they do not actively support 

Case 15, Design a fixture for the arc welding of the 
structure shown in Fig. 20-37. 

The structure is a rectangular frame with two 
cross bars. The frame consists of 6 by 2 1/2-inch 
(150 by 65-mm) angles, bent from 1/2-inch (13-mm) 
-thick plate. The cross bars are 6 by 7/16-inch 
(150 by 1 1-mm) flats. The length of the structure 
is 6 feet (1.8 m), equal to the height of a person. 
The weight is 325 pounds (147 kg); in comparison, 
the fixture, as shown in Fig. 20-38, weighs, empty, 
428 pounds (194 kg). With the exception of the 
four short welds across the narrow angle flanges, 
all welds are 90 degree corner welds and they are 
parallel. Therefore, the fixture is designed to be 


4100 RCf 



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Fig. 20-37. An arc welded rectangular structure with cross 

Fig. 20-38. A welding fixture for the structure shown in 
Fig. 20-37. 

rotated primarily around one axis which will succes- 
sively bring each of these welds into a convenient 
position. As for the short welds on the narrow 
angle flanges, if the fixture is mounted on a commer- 
cial positioner, there may be at least one additional 
axis of rotation whereby the fixture can be laid 
down flat, or the part can simply be removed from 
the fixture and placed on a bench for these welds. 
Thus, it is a matter of available equipment. In the 
design shown in full lines, the fixture has a circular 
base with 6 holes for clamping it on the positioner 
table. If corner welds only are required to be done 
in the fixture, it can be provided with a shaft to be 
mounted horizontally in its own bearing bracket. 
The circular plate is retained and serves as an index 
plate for indexing the fixture to four positions 90 
degrees apart. This alternate solution is indicated by 
chain-dotted lines. 

The fixture is a rectangular frame with diagonals, 
provided with standing lugs that function as locators 
for the parts, and with push-pull toggle clamps for 
securing the four frame sides in position. The loca- 
tors for the cross bars are welded on the diagonals 
in pairs, with sufficient space between for receiving 
and locating the parts. In this way, their position 
is fully defined, and no clamping is needed. Each 
clamp acts in a point, halfway up on the locator, 



Ch. 20 

so that the correct orientation of the frame members 
is secured when the parts are installed. The end 
members and the cross bars are positively positioned 
between the side members; the side members can 
slide longitudinally and be visually lined up, relative 
to the end members, at the corners. Thermal dis- 
tortion is not significant, as the welds are small in 
relation to the mass of metal, and are located far 
apart. The effect of the heat is expansion, and the 
result is essentially a bending of the frame members 
away from the locators over the free spans between, 
and outside of, the clamped points. This distortion 
is slight, transient, and not harmful. The shrinkage 
distortion which tends to pull the parts together is 
effectively resisted by the wide and massive locators. 
All locators are rectangular blocks of substantial 
thickness. They could have been designed as T- 
section brackets, saving some steel, but would cost 
significantly more in cutting, fitting, and welding. 
The fixture body consists of square tubing, 4 by 4 
inches ( 1 00 X 1 00 mm), 1 1 gage (. 1 20 inch or 
3.05 mm). With the weight of the part and the 
fixture, the required strength and rigidity might well 
be obtained with smaller dimensions, or with round 
tubing. However, the use of this size of square tub- 
ing provides large, flat areas for supporting the 
pans and permits assembly without the use of gusset 
plates. The width of the tubes provides areas for 
the mounting and welding of the locators. The two 
longitudinal tubes are made one inch longer than the 
part, to provide backing for the welding across the 
narrow angle flanges. The total weight of the tubes 
is 70 pounds (32 kg), in other words, only a small 
fraction of the total weight of the fixture. The pads 
for the clamps are flat plates. The width may seem 
somewhat excessive, relative to the base of the 
clamp, but again, the extra width provides the 
rigidity which otherwise would have required a 
bracket with a rib; a more expensive design. In the 

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design of welding fixtures, it is often possible to 
economize by trading off labor cost for cutting and 
welding, against some additional material. 

All dimensions shown in Fig. 20-38 are REF 
(untoleranced) dimensions. As an exercise, calculate 
the dimensions with their proper tolerances so that 
the fixture will produce the part with the tolerances 
shown in Fig, 20-37, 

Some positioners, particularly those for automatic 
welding, have developed into full-fledged machine 
tools, usually of the lathe type. A welding lathe 
carrying an internal fixture is shown in Fig. 20-39. 
One segmented and collapsed backing ring is seen* in 
the photograph. The part, a thin-walled cylinder, 
has runner rings on the outside, supported in the 
four large frames, which function as steady rests. 

There is no physical upper limit to the size of weld- 
ing equipment. Probably the largest existing welding 
positioner is shown in Fig. 20-40. It weighs 200,000 
pounds (91,000 kg) and is designed for use at the 
U.S. Naval Shipyard on Mare Island. It rotates the 
work at speeds ranging from 0.052 RPM (19,2 
minutes for one revolution) to 0.0052 RPM and in 
four minutes can tilt the table 60 degrees from the 
horizontal. The table diameter is 33'o" (10 m); 
the height, measured to the table in the horizontal 
position, is 20'l0" (6.35 m), and its work capacity 
is 1 50,000 pounds (68,000 kg). 

Courtesy of Aerojet-General Corp. 
Fig. 20-39. A welding lathe with external steady rests. 

Courtesy of Pandfiris Weldment Co. 
t'ig. 20-40. A 3 3 -foot-diameter welding positioner. 



Universal and Automatic Fixtures 

Definition of "Universal" 

The term "universal fixtures" covers two different 
types of equipment. The first type consists of a 
drill jig body with a quick-acting clamping and 
locking mechanism, and which can be provided with 
interchangeable drill bushing top plates and sub- 
bases (adapters) to support the work. The second 
type comprises sets of building elements which can 
be temporarily assembled to a fixture and disman- 
tled after use. Both types are available from com- 
mercial sources, but they can also be designed and 
built "in-house" to advantage. Examples of sim- 
plified designs for this purpose are included in the 
following sections. 


Custom-made Jigs 

The merits of the machine tool vise (and other 
vises) for use as fixture bases have been described 
at length in other chapters. Examples of how vari- 
ous types of vises, not only machine tool vises, can 
be converted to universal drill jigs were shown in 
Figs. 18-35 through 18-37. The vise drill jig shown 
in Fig. 8-36 includes, in addition, two V-blocks and 
demonstrates a principle used in universal drill jigs 
of a simple type for cylindrical parts. It differs from 
the more commonly used type by having the V-block 
installed with horizontal Vs. The usual type con- 
sists of a V-block with a vertical V, one or more 
brackets, each with a drill bushing centered in the 
axis of symmetry of the V, a clamp, and an end stop. 
It is used for drilling holes along a diameter of cylin- 
drical parts within the full range of diameters that 
can be accommodated in the V-block. 

Pump Jigs 

The most common type of universal drill jig is the 
pump jig, so named because it is operated by a pump- 

ing movement of the operating handle. A pump 
jig, designed and built "in-house" is shown in Fig. 
18-42. In the clamping position the top plate A with 
the drill bushing is held against the work by spring 
pressure. When the handle D is lifted, it lifts the top 
plate and releases the work. 

A commercial drill jig with three posts is shown 
in Fig. 21 -la. The construction of universal drill jigs 
is quite simple. The outer style and some details 
may vary, but the principle remains the same. The 
top plate is secured to either one, two, or three verti- 
cal posts. The posts are raised and lowered through 
a lever arm, with the top plate maintaining a hori- 
zontal position at all times. The length of travel (the 
clamping range) of the top plate is quite limited, 
normally about 25 percent, or even less, of the maxi- 
mum opening height. For most workpieces it is 
therefore necessary to provide a sub -base, known as 
an adapter, to lift them up so that the top of the 
work comes within the clamping range, as shown in 
Fig. 21-2. Locators, attached to the top plates or to 
the adapters, are also used. A few rules can be 
formulated for the design of the adapters and loca- 
tors. Assuming that the part has one machined 
and one unmachined side, horizontal alignment is 
established from the machined surface. When the 
holes must be drilled from the machined surface, 
this side must be up, and to ensure full alignment 
with the bottom side of the top plate, the adapter 
must be made much smaller than the surface of the 
part. This condition is shown in the illustration. 
When the holes must be, or can be, drilled from the 
unmachined side, the machined side is down and the 
adapter is made large enough to align and support it 
on its entire width. 

Internal location is preferred to external location. 
A concentric locator is attached to that member 
from which horizontal alignment is established. Pref- 
erably, a concentric internal locator is attached to 
the top plate, but a concentric external locator is 
attached to or made integral with the adapter. 




Ch. 21 

Locators are designed with a short locating surface 
to prevent jamming, and with conical or otherwise 
tapered lead surfaces (pilots). Locators on the top 
plate have long lead surfaces so that the operator can 
see that they catch the part. It may be necessary to 
machine a clearance space in the adapter if the lead 
surface is longer than the height of the part. Loca- 
tors on the adapter have short leads because they are 
visible to the operator when the jig is open. A long 
lead would also require that the part be lifted higher 
when inserted and removed. In either case, there 
must be space enough over or under the locator in 
the open position to bring the part in and out. 

Parts having fully concentric configurations (con- 
tours and holes) do not require radial location, i.e., 
components that prevent rotation away from the 
correct position. Most noncircular parts require ra- 
dial locators, usually hardened steel blocks, fastened 
to the top plate or the adapter. 

opposite end is for locking the top plate in the 
fully open position. View c shows a unit consisting 
of a pinion and rack set mounted in a bearing bush- 




I ! I I 

Fig. 21-2. General arrangement of the work, top plate, and 
adapter in a pump jig. 

a c 

Courtesy ofJergens Inc. 
Fig. 21-1. a. Sectional view of a pump jig with three posts, showing the rack and pinion movement, b, A pinion shaft with 
integral braking cone. c. A rack and pinion locking unit with bearing bushing and operating handle. 

There is generally a locking mechanism connected 
with the operating mechanism. This device main- 
tains the clamping pressure and prevents the work 
from shaking loose when it is drilled. One widely 
used type of operating and locking mechanism is 
shown in Fig. 21 -la, b, and c. As seen in the sec- 
tioned view a, the jig contains a helical gear pinion 
and a mating rack which is integral with the post. 
The pinion shaft (see b) carries the operating handle 
and at one end an integral brake cone. A counter 
cone is fitted on the opposite end, and mating coni- 
cal seats are machined into the base of the jig. 
Operating the handle closes the jig, and as clamping 
pressure builds up, an axiai thrust is developed which 
locks the brake cone into its seat. The cone at the 

ing which contains the conical seats. Such units are 
commercially available for installation in custom 
designed jigs. 

Other types of pump jigs employ braking devices 
based on the principle of the overrunning clutch, as 
in a bicycle wheel, or a pair of cam operated brake 
shoes. Locking units of these types are also com- 
mercially available. 

The jig shown in Fig. U-3 has several refinements. 
The top bushing plate is interchangeable and ad- 
justable, the adapter for the work is a V -block, and 
an adjustable end stop is provided for locating the 
work. A drill jig with an air-operating clamping 
mechanism is shown in Fig. 21-4. Some drill jigs 
have a fixed top plate, and the adapter is mounted on 

Ch. 21 



Courtesy of Anton Ruckert, Berlin, Germany 
Fig. 21-3. Pump jig with V -block and interchangeable and 
adjustable bushing plate. 

a post that can be raised for clamping the work. 
Some drill jigs have the clamping area located be- 
tween the posts while still others provide the feature 
of improved access to the clamping area by allowing 
the top plate to swing 1 80 degrees out of the way in 

Courtesy of Heinrich Tools Inc., Racine, Wis. 
Fig. 21-4. An air-operated drill jig. 

the horizontal plane or tilt 45 degrees in the vertical 
plane. Some jigs have the rear side of the body pre- 
cision-machined square with the bottom surface. In 
this way the jig becomes a tumbling jig; it can be laid 

down on its back to permit the drilling of holes in 
two directions, 90 degrees apart. 

Cast iron top plates, fitted to the posts, but with- 
out bushing holes, are supplied by jig manufacturers. 
Blanks for top plates can also be economically pro- 
duced by torch cutting them out of steel plate and 
drilling and reaming the post holes with a simple 
drill jig, A commercial punch holder, i.e., the upper 
half of a postless die set, makes a satisfactory and 
inexpensive adapter blank. 


Speed of operation is the greatest advantage of the 
universal drill jig. It is so significant that the use of 
this type of jig as a permanent component of a 
single-purpose tool can be economically justified in 
highly repetitive work. 

The jig operation is fast since it is operated by a 
single sweep of the lever handle, which eliminates the 
need for loose tools and clamping parts. This man- 
ual operation is always the same, regardless of the 
part configuration, and a line of dissimilar parts is 
drilled as if they were all alike. The rate setting can 
be done without individual time studies, by deter- 
mining and recording the handling times, once and 
for all, and calculating drilling time from speed and 
feed. The jig is loaded and unloaded in the upright 
and open position, and does not have to be turned 
over as do most other drill jigs. When one hole only 
is to be drilled, the jig can be secured to the drill 
press table; therefore, the drill will enter the bushing 
practically without touching, which results in pro- 
longing drill and bushing life. Top plates and adap- 
ters can often be so designed lhat chip cleaning is 
greatly simplified if not completely eliminated. 

The cost of a fop plate with adapter is generally 
less than the cost of a complete single-purpose drill 
jig, and top plates and adapters are interchangeable 
so that the main body and operating mechanism can 
be used for a variety of jobs. A top plate is usually 
more expensive than an adapter, although the ma- 
terial costs are a minor consideration. The greatest 
expense item is the precision boring of the post and 
bushing holes. A top plate may be equipped with 
bushings for more than one hole configuration, and 
an existing top plate may be modified by the addi- 
tion of more bushings. Different parts with the 
same hole configuration can be accommodated by 
changing adapters and locators. If a top plate is 
made with integral locators, then it can be turned 
over and the other side used. In such cases it may 
be necessary to use headless drill bushings. 



Ch. 21 

Chips and Coolant Considerations 

When coolant is needed it is directed onto the top 
plate. Commercially available cast top plates are 
formed as trays and provide a reservoir from which 
the coolant flows down along the drills. For use 
with a flat top plate, a ring large enough to encom- 
pass all the drills used is cut from 1/2-inch (13-mm) 
steel plate, and is laid on the top plate to hold the 
coolant. Chips are swept off by simply sliding the 
ring over the plate. 



^ ■ o ex© ojf © 

Commercial Universal Fixtures 

A simple, yet quite versatile and efficient fixture 
is shown in Fig. 21-5. It is in essence a glorified 
V-block. With the clamping screws shown, it can 
hold parts of any configuration within its own di- 
mensional limitations. It can be rotated (like a 
tumble jig) 45 degrees and 90 degrees in its own 
vertical plane, and rotated 90 degrees to either side. 
A swivel base is available by which it can be rotated 
at an arbitrary angle. It can be used in any machine 
tool, including the lathe, where it is clamped on the 
face plate. 

Courtesy ofSchwenzer Tool & Die Co., Inc., Buffalo, N.Y. 
Fig. 21-5. Simple and versatile universal fixtures. 

A different and more representative type of fix- 
ture is shown in Fig. 21-6. The principal compo- 
nent is the base plate. Edge strips are bolted on, so 
that a lying V-block is formed, and a part is clamped 
in this V by means of clamping screws of the same 
type as those shown in Fig. 21-5. It is used here 
for precision drilling. The drill bushing, mounted in 
a large boss, is located from the sides of the V by 
means of gage blocks. 

Courtesy of Montgomery and Co. 
Fig. 21-6. A base plate type of universal fixture. 

The backbone of every universal fixture is the 
sub-base; the various systems differ in the types 
and number of components. The elements in gen- 
eral are of steel, hardened and ground to tolerances 

Adaptor Block 
Thrust Element 

(Stop Element 

Courtesy of "Machinery " Magazine, London, June 20, 1946 
Fig. 21-7. Universal fixture components; stop-and- thrust 

Ch. 21 



Right Angle Sliding Plate 

Eccentric Pin 

Stop Elements 

Courtesy of "Machinery" Magazine, London, June 20, 1946 
Fig. 21-8. Universal fixture components; adjustable location 

of the order of 0,0003 inch (0.008 mm) on the 
significant dimensions. The basic elements are rec- 
tangular blocks with T-slots, called stop elements 
{manufacturer's terminology), thrust elements, and 
adapter blocks with bolt holes for buttressing the 
stop elements, fixed and adjustable height elements, 
angular elements with fixed angles of 30, 45, and 
60 degrees, adjustable angular elements (including 
sine bars), special elements for the attachment of 

Courtesy of "Machinery" Magazine, London, December 27, 

Fig. 21-9. Universal fixture components; miscellaneous 

locating pins (also adjustable by means of an eccen- 
tric), V-blocks, jack screws, clamps, holders for drill 
bushings, and bearings for boring bars. Straight and 
angle straps are provided for the joining of two sub- 
bases, and for bracing stop elements, for example, 
for forming rigid corners. Sub -bases, with T-slots of 
standard dimensions and spacing, are made of nickel 
cast steel and are available in square, rectangular, 
and round shapes. Typical elements are shown in 
Figs. 21-7, 21-8, and 21-9, and a completed milling 
fixture is shown in Fig. 21-10. 

Serrated Pads 
Double Swivel Clamp 

Bled and Screw-. 


Height Element 
Stop Element 

Sub -Base 

Courtesy of "Machinery" Magazine, London, December 27, 

Fig. 21-10. A milling fixture built with universal fixture 

Such a fixture is not designed in advance but is 
built up with dummy blocks made of a castable 
plastic material around an actual production part, 

Courtesy ofMultijig Ltd.; Tyne Valley Tool and Gage Co., 

Northumberland, England 
Fig. 21-11. A drill jig built with universal fixture 



Ch. 21 

or a replica of a part. When the dummy fixture is 
completed, it is photographed in detail. The photo- 
graphs are used in the toolroom for assembling the 
actual fixture, and provide a permanent record for 

Another system uses holes instead of T-slots for 
the assembly. The holes are alternately straight pre- 
cision holes and tapped holes and are closely spaced 
in a modular pattern. A drill jig built with com- 
ponents from this set is shown in Fig. 21-1 1. The 
jig was built in two hours and is used for drilling and 
reaming holes with 0. 003-inch (0.08 mm) tolerance 
on the center distances. 

Custom-made Universal Fixtures 

An experienced fixture designer in cooperation 
with a good toolmaker can make any desired type 
of universal jig or fixture. An example of a universal 
jig construction is shown in Fig. 21-12. It is known 
as a tool maker's universal drill jig and consists of a 
heavy plate A, containing adjustable locating rods 
B with locking screws, and a boring for interchange- 

'■-' . ; £ 

B B 

Pig. 21-12. The toolmaker's universal drill jig. 

Fig. 21-13, An application of the toolmaker's jig, 

able drill bushings C, for different hole sizes. The 
application of this jig for drilling and reaming the 
holes in a large die block is shown in Fig. 21-13. 
Parallels are clamped to the edges of the die block, 
and the jig is positioned against the parallels with 
the locating rods; measurements are taken with mi- 
crometers and gage hlocks. When the bushing C, is 
correctly positioned, the jig is clamped to the die 
block and the hole is spotted, drilled, and reamed. 
The procedure is repeated for each hole. 

A drill press can be converted to a makeshift jig 
borer by installing a compound table with slides at 
right angles, on the drill press table. An upright with 
a bracket carrying a liner bushing for the insertion 
of different size slip bushings is installed with the 
bushing axis in line with the drill spindle. The 
slides arc positioned from fixed stops by means of 
micrometer gages, gage blocks, or gage bars. 

In developing and building a universal fixture set, 
the first task is to design the sub-base. Any base 
plate with parallel T-slots or mounting holes will 
serve the purpose, but a design with partly diagonal 





Courtesy ofE, Tfiaulow 

Fig. 21-14. A universal fixture sub-base. 

Ch. 21 



ribs and T -slots, such as that shown in Fig. 21-14," 
has advantages over the conventional type. The T- 
slot pattern is more versatile, and diagonal ribs pro- 
vide additional rigidity against torsion. 

Some angle plates with single (Fig. 21-15)* and 
multiple T-sIots and some smaller and larger tooling 

Courtesy ofE. Thaulow 
Kig. 21-15. Angle plate with a single T-slot for use in a 
universal fixture. 

blocks (Fig. 21-16) are added to the base. There is 
no rule that forbids the use of T-slots and mounting 
holes within the same set. Each system has its ad- 
vantages and selection is made according to what is 
needed. Finally, an assortment of bushings, clamps, 
bolts, and sundry items is selected from fixture com- 
ponent catalogs, and the universal fixture set is ready 
for its first assignment. 






Courtesy of Challenge Machinery Co., Grand Haven, Mich. 
Fig. 21-16. A large tooling block for use in a universal 

--— — . - !*irs — — - C-- 

1 E. Thaulow, Maskinarbefde (Copenhagen: G.E.C. Gad's 
Forlag, 1930) vol. 11. 

2 E. Thaulow, Maskinarbefde (Copenhagen: G.E.C. Gad's 
Forlag, 1928) vol. 1. 


Definitions and Principles 

Automatic fixtures are those in which the part is 
clamped and undamped by the use of a power medi- 
um, usually compressed air (pneumatic fixtures) or 
oil under high pressure (hydraulic fixtures). These 
devices are used for five purposes: 

1. To apply a greater and more consistent clamp- 
ing force than is possible by manual operation 

2. To reduce operating time and operator fatigue 

3. To operate the fixture by remote control (in- 
cluding foot operation) 

4. To clamp simultaneously and uniformly in mul- 
tiple fixtures 

5. To be able to incorporate the fixture into an 
automated program (transfer machines, convey or- 
ized production lines, or numerical control [N/C] 
machine tools). 

The actuating member is always a cylinder with a 
piston or a plunger (referred to in the following 
as the power cylinder). The actuating force is applied 
directly or indirectly; direct actuation means that 
the force from the power cylinder acts directly on 
the part or on a clamp that is in contact with the 
part; indirect actuation means that the force from 
the power cylinder acts on the clamping element 
through a kinematic chain, which can be a linkage 
system or a combination of cams and links. 

Common Features and Advantages of Pneumatic 
and Hydraulic Fixtures 

There is no general preference for selecting one 
system over the other. It is not even possible to 
predict relative costs without making comparative 
estimates. For the selection of a system, the follow- 
ing guidelines can be applied: 

Hydraulic fixtures utilize significantly higher work- 
ing pressures than pneumatic fixtures. Hydraulic 
fixtures are preferred, therefore, where large clamp- 
ing forces and short strokes are required and where 
available design space is limited. Permissible flow 
velocities in conduits are 15 feet per second (4.6 m 
per sec) in pressure lines and 4 feet per second (1.2m 
per sec) in other lines. The maximum piston speed 
is 2 feet per second (0.6 m per sec). Hydraulic 
fixtures operate satisfactorily under average cycling 
conditions and at normal temperature levels. The oil 
temperature must not exceed 140F (60C). At higher 
temperatures oils lose viscosity, oxidize, and, in the 
long run, break down. Mating parts within the 
equipment that expand differently, may bind or 



Ch. 21 

leak. Hydraulic cylinders are self-lubricating; air 
cylinders are not. 

Pneumatic fixtures are used where medium clamp- 
ing forces with almost any length of stroke are re- 
Quired, and where ample design space is available. 
Permissible flow velocities in conduits are signifi- 
cantly higher for air than for oil; pistons in air 
cylinders operate at speeds ranging from 1/4 inch 
per second (6 mm per second) to 1 feet per second 
(3 m per sec) with 2 to 3 feet per second (0.6 to 
0.9 m per sec) as a commonly used average; this 
means that the clamping operation is practically 
instantaneous. Pneumatic fixtures operate satisfac- 
torily under conditions of high cycling and elevated 

The use of a power medium (air or oil) under 
constant pressure permits close control of the clamp- 
ing pressure which, first, ensures that the part is 
sufficiently gripped and, second, reduces the risk 
of distortion or even breakage of the part. The initial 
cost of pneumatic and hydraulic clamps is higher 
than the cost of manually operated clamps; they 
also incur some operating expenses (compressed air, 
power for the hydraulic pump), but these are insig- 
nificant. Air is cheap. A representative figure for 
the cost of compressed air in a factory is $0.12 to 
SO. 15 per 1000 cubic feet ($0.42 to $0.55 per 
100 m 3 ), and it takes many piston strokes for a 
small cylinder to consume one cubic foot. 

The dominating, if not decisive factor, in favor of 
using automatic fixtures is the saving in labor costs. 
The saving is about 80 percent on manual clamping 
operations of up to 1/2 minute duration, and 85 
percent on longer operations. In addition, operator 
fatigue is virtually eliminated. 

Each of the two systems can be manually or 
automatically actuated by electrical controls (sole- 

Normally, the power medium is applied to the 
clamping operation. The release of the clamp and 
the return to the open position can be accomplished 
by application of the power medium in the opposite 
direction, by a return spring, or by a combination of 
the two. Where several clamps are used in the same 
fixture, they can be timed to function simultaneous- 
ly or in a predetermined sequence. The force F 
(pounds) exerted by a power cylinder of diameter 
Dp (inches) with a piston rod diameter Dr (inches), 
and an operating pressure P pounds per square 
inch is calculated by 

F = 0.7854 (Dp 2 ~D R 2 )XPX (0.85 . .. 0.90) 

For that side of the piston where there is no piston 
rod, Dft = 0. The factor (0.85 . . . 0.90) is the 
mechanical efficiency. 

In Metric units P is in newtons, Dp and Dr are in 
millimeters, and P is in newtons per square 

Pneumatic Fixtures 

Pneumatic fixtures are operated with air from the 
compressed air supply system in the plant. The oper- 
ating pressure is nominally 100 pounds per square 
inch (0.69N per mm 2 ). It is usually assumed to 
range between 80 and 100 pounds per square inch 
(0.55 to 0.69N per mm 2 ), but may well drop to 40 
to 50 pounds per square inch (0.28 to 0.34N per 
mm 2 ) at points at a great distance from the source, 
or in cases where the compressor or the distribution 
lines are overloaded. Air cylinders are available with 
diameters up to 14 inches (350 mm) and lengths up 
to 3 to 4 feet (0,9 to 1 .2 m). The larger sizes are not 
for clamping purposes, but are used for moving the 
part into or out of the fixture, for rotating the 
part, or for moving the fixture from one station to 
another, etc. 

To ensure constant output from the power cylin- 
der and constant clamping pressure in the fixture, 
the input pressure must be maintained constant and 
independent of pressure fluctuation in the air supply. 
This is done by means of a pressure reducing valve 
in the supply line to the fixture. 

With constant power output, the power cylinder is 
essentially an elastic system. It maintains its own 
force as the clamping force on the clamped part and 
it holds the clamped part in a position of stable equi- 
librium as long as the clamping force is superior to 
any opposing force. However, if the opposing force 
temporarily or permanently equals the clamping 
force, the equilibrium is no longer stable, and if the 
opposing force, even only temporarily, exceeds the 
clamping force, the clamping element is pushed back 
and the part is likely to be thrown or pulled out. 

For the stability of the operation, there must be 
provided some means by which release of the clamp 
is prevented if the air supply is cut off or the supply 
pressure falls below the pressure at which the reduc- 
ing valve can maintain constant operating pressure. 
One method is to insert a pressure switch in the 
supply line which will stop the machine if the pres- 
sure drops below a safe limit or the supply is cut off. 
A different and frequently used method is to trans- 
mit the force from the power cylinder to the clamps 

Ch. 21 



through a mechanical device, a kinematic chain, 
which is self-locking when it is in the clamping posi- 
tion. Wedges, cams, and toggle joints are used. Once 
fully activated, they hold the clamps engaged, even 
if the air pressure vanishes. The introduction of a 
kinematic chain for the transmission of the force 
eliminates the elasticity in the system and has the 
additional advantage that it is now possible to in- 
crease the applied clamping force; in mechanical 
language this means to introduce a mechanical ad- 
vantage greater than one. For a given required 
clamping force this permits the use of a smaller 
power cylinder or the use of a lower operating pres- 
sure. A self-locking device normally has a mechani- 
cal advantage that is significantly greater than one. 

Air Cylinders 

Rotating power cylinders for the actuation of 
various types of chucks are commercial items used 
on many semiautomatic and automatic lathes. Non- 
rotating air cylinders are used on commercially 
available vises. These devices are workholders, not 

A universal drill jig equipped with an air cylinder 
is shown in Fig. 21^. Many commercial clamps 
are built with an attached or integral power cylinder, 
mostly for air operation, some of them capable of 
air operation and oil operation in the same cylinder 
(dual pressure clamps). 

Air cylinders are available from many sources and 
with a variety of rod end and rear end accessories as 
shown in Fig. 21-17, for attachment to the fixture 

Courtesy of Parker-Hannifin Corp, 
Fig. 21-17. Air cylinder with different types of connecting 
components (end accessories). 

and the clamp. A power cylinder can perform the 
following functions; push, pull, raise, and lower. 
Cylinders are single-acting and double-acting. They 
can be supplied with single-end and double-end pis- 
ton rods, protruding from one end only, or from 
both ends of the cylinder. For cylinders with 
single-end piston rods, where possible, clamping with 
pressure on the piston-rod side of the piston, and 
releasing with pressure on the full piston area are 
recommended. The additional release force may be 
needed to overcome any jamming or sticking in the 
clamps. When dimensioning air cylinders a length 
that is two times the net calculated length of travel 
plus the length of the piston is recommended to 
compensate in advance for future changes in work 
dimensions and for other unforeseen changes. 

An air-operated clamping device with integral 
power cylinder is shown in Fig. 21-18. It illustrates 
most of the previously described principles. The 





Courtesy of Cincinnati Mitacron Inc. 
Fig. 21-18. An air-operated clamping device with integral power cylinder and a sloping cam. 



Ch. 21 

actuating element is the large horizontal plunger 
which carries the piston at its right end and a sloping 
double-acting cam at its middle portion. This cam 
engages a small vertical plunger which actuates the 
clamp through spherical equalizing washers. The 
operation is controlled by an air valve. For clamp- 
ing, air is admitted to the small area on the left side 
of the piston, and moves the large plunger to the 
right. The sloping cam pushes the small plunger 
down and clamps the part. The inclination angle of 
the sloping cam surfaces equals the friction angle for 
dry surfaces so that the clamp is locked in position 
even without the help of cylinder pressure. In order 
to release the clamp, air is admitted to the full 
piston area on the right side of the piston, which 
moves the large piston to the left. The initial 
movement simultaneously releases the clamping pres- 
sure and the frictional forces, and further movement 
to the left causes the upper sloping cam surfaces to 
engage the vertical piston and lift it from the work. 

Hydraulic Fixtures 

The acceptable working pressure for a hydraulic 
fixture is determined by the fixture, the conduits, 
and the hydraulic power source. The design pressure 
for hydraulic fixtures is, in a sense, arbitrary, be- 
cause a fixture can theoretically be designed for any 
desired pressure level. However, higher pressures 
require not only larger material dimensions, but also 
tighter fits, closer tolerances, and smoother (pre- 
cision lapped) surfaces to ensure against leakage. As 
explained in Chapter 1 1 , the practical upper limit 
for the working pressure is 15,000 pounds per 
square inch (103N per mm 2 ) when a plastic (PVC) 
is used as the pressure medium, and 10,000 pounds 
per square inch (69N per mm 2 ) when oil is used. 
Many hydraulic fixtures and fixture components are 
designed for significantly lower pressures. The dual 
pressure clamps referred to in the previous section 
are designed for operation with air pressures of 1 00 
to 250 pounds per square inch (0.7 to 1.7N per 
mm 2 ) or oil pressures up to 500 pounds per square 
inch (3.4N per mm 2 ). 

The maximum working pressure for standard hy- 
draulic tubing and fittings is about 6000 pounds per 
square inch (4 IN per mm 2 ) for stationary pressure 
lines and about 3500 pounds per square inch {24N 
per mm 2 ) for flexible tubing. Special types and 
qualities are available for higher pressures. 

Hydraulic fixtures can be powered from the ma- 
chine tool, if it has a hydraulic drive, from a separate 
hydraulic pump, and from a pneumatic-hydraulic 
booster (pressure intensifier). A machine tool with 

a hydraulic drive is a convenient and inexpensive 
power source for the fixture. The operating pres- 
sures used in machine tools are low (1000 pounds 
per -square inch [7N per mm 2 ] or less) compared to 
the previously quoted values, and the fixture de- 
signer must always carefully check the available 
pressure. Separate and individual hydraulic pumps 
are available with almost any desired pressure, but 
the cost is frequently too high for consideration. The 
preferred power source, which is now widely used 
for fixtures, is the booster. It is simple, small, inex- 
pensive, and versatile and is commercially available. 
The booster consists of two coaxial cylinders with 
different diameters and a common piston. A low- 
pressure medium is applied to the large piston area 
and a high pressure is developed on the medium 
in the small diameter cylinder. The low-pressure 
medium is primarily air from the compressed air line 
(at around 100 pounds per square inch (0.7N per 
mm 2 ]) or medium pressure oil, for example, sup- 
plied from the hydraulic system in the machine tool, 
A booster is characterized by the area ratio, the 
maximum output pressure, and the volume of high- 
pressure medium supplied per stroke. Representa- 
tive ratios are 7:1, 15:1, 28:1, and 30:1. Output 
pressures from commercial units are 1500, 3000, 
and 7000 pounds per square inch (10, 20, and 50N 
per mm 2 ). They supply from 1 to 14 cubic inches 
(16 to 230 cm 3 ) of high-pressure oil per stroke. 
Some models are provided with an adjustable relief 
valve and provide infinitely variable pressures of 
from 1500 to 3000 pounds per square inch (10 to 
20N per mm 2 ). Boosters are smalt and since they 

Proprietary to Tomco, Inc. 
Fig. 21-19. A fixture with a hydraulic clamp and with the 
air/ hydraulic booster mounted on the machine 

Ch. 21 



require only an air connection, they can be placed 
close to the fixture. It is often found convenient 
to mount the booster on the machine tool as shown 
in Fig. 21-19, 

Power cylinders are designed for and rated at max- 
imum working pressures of 3000, 5000, and 10,000 
pounds per square inch (20, 35, and 70N per mm 2 ) 
and are commercially available with up to approxi- 
mately 8000 pounds (36 kN) maximum capacity. 
Matching holding brackets and complete clamping 
sets are also available. The most recent design trend 
is characterized by small dimensions and low profiles. 
The smallest hydraulic power cylinders are threaded 
on the outside with thread dimensions down to 1/2- 
13 UNC (see Fig. 21-20) and provide 150 pounds 
(0.7 kN) of force at 3000 pounds per square inch 
(2 IN per mm 2 ) rating with 1/8 inch (3 mm) plunger 
travel. A typical low profile clamping unit is shown 
in Fig, 21-21. When power cylinders are incorpor- 
ated in individual fixtures, utilizing only 75 percent 
of the rated maximum stroke to provide a reserve for 
workpiece variations, etc., is recommended. 

An entirely different principle for hydraulic clamp- 
ing consists of using a thin-walled sleeve expanded 
by hydraulic pressure into the part that is being 
clamped. An application to a lathe fixture is shown 
in Fig, 21-22. The fixture is completely self-con- 
tained and works only with the pressure medium (oil 
or plastic) that is confined in the cavities inside the 
sleeve. Pressure is applied from the rear end of the 
lathe spindle by means of the actuating rod located 
in the axis of the fixture. The principle is the same 
as that used in the fixture shown in Fig. 1 1-16. The 
sleeve expands very uniformly , except near the ends, 
and the pressure on the part is also very evenly 
distributed. The unit pressure is relatively low, and 
the part is not distorted. Accuracy on the clamping 
surface can be maintained within a toierance of 
0.0002 to 0.0004 inch (0.005 to 0.010 mm) TIR, 

Fig. 21-21 

A drill jig with 
hydraulic clamps. 

Proprietary to Tomco, Inc. 
two law-profile swinging 

depending on the expansion. With a design stress 
of 60,000 pounds per square inch (400N per mm ! ) 
in the sleeve, the maximum expansion is 0.002 times 
the sleeve diameter. This type of fixture is built 
in sizes from 1/2-inch (13-mm) diameter minimum 
and can be used on the inside and outside of cylin- 
drical and slightly tapered surfaces (up to a 6 degree 
taper angle), on stepped diameter cylinders, and for 
the clamping of several shorter rings on or in one 
fixture, as indicated in the illustration. It may be 
noted that when larger expansion is needed, the 
sleeve can be made of nylon with steel inserts. 

I s y 






Proprietary to Tomco, Inc. 
Fig, 21-20. Small (miniaturized) hydraulic power cylinders. 

Proprietary to Hydra-lock Corp. (Pat. Pend.J 
Fig. 21-22. An arbor- type lathe fixture with hydraulically 
expanded sleeve. 


Fixtures for N/C Machining 

Because N/C machining is characterized by a high 
hourly burden (overhead rate), several times higher 



Ch. 21 

than that used for conventional machine tools, a re- 
duction in non-cutting time is of first importance. 
One way to reduce the non-cutting time is by im- 
proved fixturing. 

The principle of N/C machining has drastically 
affected the fixture design. A common type of 
N/C machine, the N/C machining center, performs 
milling, drilling, tapping, boring, and reaming in one 
setting of the part and reduces the number of fix- 
tures from two or three, to one. The need for indi- 
vidual cutter guides, except for the starting position, 
is eliminated because all subsequent positioning of 
the cutters relative to the work is covered in the 
programming for the operation. Fixtures for N/C 
machining are thereby greatly simplified, in fact, 
they are reduced to locating and clamping devices 
for the workpiece. 

Most N/C machines have what is called a "floating 
zero" which allows the programmed starting point to 
be adjusted to the actual starting point on the part. 
This point can be defined by a single tool setting 
block on the fixture or by a point selected on the 
part itself. This initial operation is not always 
necessary. Several types of N/C machines work with 
preset tools, and when the fixture is accurately 
located relative to the machine, it is also positioned 
relative to the cutters. Edge locators with a close fit 
in the slots in the machine table are provided for 
this purpose. Horizontal and vertical universal fix- 
ture bases are also available for most machines. An 
error in the mounting of a fixture endangers the 
operation, because the cutter may collide with the 
fixture body. Machines with more than one slide 
motion have interference zones for the protection 
of vital parts, as for example, an index table. Only 
one slide at a time is permitted in an interference 
zone. To fully utilize both slide motions on a part 
held in a fixture, it is sometimes necessary to lift the 
fixture above the interference zone by mounting it 
on a raiser block of sufficient height. 

N/C fixtures must be strong and rigid to ensure 
correct part tolerances and have provision for quick 
loading and unloading of the workpiece. Weight 
considerations are unimportant when the fixtures are 
indexed from position to position and are moved on 
and off the machine by mechanical means. 

One Tesult of the simplified design is that there 
is easier access for the part to the interior of the 
fixture, and the handling of the part is correspond- 
ingly simplified and facilitated. 

A simple and easily fabricated universal fixture 
base, suitable for short -run production, consists of a 
plate with key seats, T -slots, precision holes, and 

tapped holes in a regular pattern. Keyseats and T- 
slots are spaced at 6-inch (150-mm) intervals, and 
holes are located with 3-inch (75-rnm) center dis- 
tances. This pattern permits side and end locators 
(strips and buttons) to be installed for any con- 
ceivable configuration of the outline of the part. It 
permits easy handling of the part and it provides the 
possibility for locating all necessary clamping de- 
vices. For fast operation, the clamps are hydra uli- 
cally operated and all power cylinders are fed from 
the same source of high-pressure oil. In this way, 
all clamps are actuated simultaneously and with the 
same pressure, yet still independently so that they 
automatically equalize for any variations in part di- 
mensions. The source of high-pressure oil can be a 
booster, but since the total oil volume is relatively 
small, it is more practical to use a hand pump. When 
the oil volume required is small enough, a screw 
pump will suffice. The uniformity in the pressure 
distribution eliminates or minimizes distortion of 
the part. The time saving obtained by the simultane- 
ous application of all clamps is repeated after machin- 
ing, when the clamps are released and automatically 
retracted, likewise, simultaneously. With automatic 
control of the hydraulic system, the operation of 
the clamps can be included in the program and 
closely coordinated with the machining cycle. 

Proprietary to Vlier Engineering Corp. 

Fig, 21-23. A typical N/C fixture setup with dual fixtures 

and low profile swinging hydraulic clamps. 

The most drastic reduction in loading time is 
realized by the use of dual fixtures. With smaller 
parts, two fixtures are mounted on the machine 
table; one fixture is unloaded and loaded while the 
part in the other fixture is being machined. With 
large parts and long machining cycles, two sets of 
fixtures and base plates are used. One set is in the 
machine, while the other set is being unloaded and 
loaded in the toolroom or on the floor. With the 
use of edge locators on the table, the exchange of 

Ch. 21 



the base and fixture sets takes little time, and maxi- 
mum machine "on-time" is achieved. 

A typical N/C setup with dual fixtures and hy- 
draulic clamps is shown in Fig. 21-23. The clamp 
straps lift and swing out automatically as the oil 
pressure is released. 

This clamp satisfies the modern requirement of a 
low profile. In N/C machining, the machine follows, 
without human supervision or interference, the path 
through which the cutter is programmed and returns 
to the starting point after a completed cut. To 
prevent collision and damage it is therefore impor- 
tant that the air space above the part is free of 

Fixtures for Transfer Machines 

The highest level of development of automatic 
fixtures is found in the fixtures that are used on or 
incorporated intoeonveyorized production lines and 
transfer machines as used in the automotive and 
other mass production industries. The design princi- 
ple for [he fixture depends on its made of operation. 
There are two modes of operation, the traveling 
fixture and the stationary fixture. 

The traveling fixture is either moved on a con- 
veyor, is an integral part of the conveyor, or is 
pushed on a track by means of reciprocating transfer 
bars with fingers that push the fixture during the 
forward stroke of the bar, and are retracted from 
the path of the fixture during the return stroke of 
the bar. The part is loaded into the fixture and 
clamped at one end station and is not released until 
the fixture has reached the other end station. At 
each work station the entire fixture is located and 
locked in position for the machining operation. 

With stationary fixtures, one at each work station, 
the part enters a fixture, is located, clamped, ma- 
chined, and released, and is then moved on to the 
next fixture. The part is moved by means of a con- 
veyor or a transfer bar. The direct use of these 
transfer mechanisms requires that the part has one 
or several flat surfaces. When that is not the case, 
the parts are nested and clamped in pallets with 
flat surfaces that can slide on the track. 

Regardless of the mode of operation, these fix- 
tures have a number of common features. The part 
is usually provided with two tooling holes for dual 
cylindrical location, and the fixture has movable 
locators, known as "shot pins." They are in a 
retracted position as the part enters the fixture; 
the part is stopped in an approximately correct posi- 
tion and is finally located as the shot pins enter the 
tooling holes. The shot pin has a tapered (conical 

or polygonal) pilot end, or a bullet nose end, or a 
combination of tapered flat surfaces and strips of 
the original cylindrical surface, as shown in Fig. 21- 
24. The tapered portion pushes the part over and 
takes the wear, and the cylinder defines the final 
position. It maintains its accuracy, because it is not 
exposed to any significant sliding motion. 

L. -- 

Fig. 21-24. The action of a tapered shot pin. 

A different method of locating is to use fixed lo- 
cators and to move the part in on an elevated plat- 
form within the fixture. After the part has reached 
its approximately correct position, the platform is 
lowered and the pins enter the tooling holes. When 
the machinery is completed, the platform is raised 
and lifts the part clear of the pins. 

Clamping devices are power operated, either pneu- 
matically or hydraulically or by means of weight-ac- 
tuated cams. Complete retraction of locating pins 
and clamps to clear the path of the part is manda- 
tory. All such moving components are therefore 
retracted by cam or gear wheel action, or by power 



Ch. 21 

(double-action hydraulic cylinders), not by springs. 
A broken or otherwise inoperative spring does not 
retract the component, and the result is serious 
damage as the motion of the part is actuated. 

A typical cam operated work station is shown in 
Fig, 21-25. The cam shaft provides the successive 
movements required to locate and clamp, and later 
unclamp, the pallet, and to actuate the various work 
slides in the station. 

Courtesy of The Cross Co. 
Fig, 21-25. Diagram of a cam-operated work station in a 
transfer line. 

The various machining operations are performed 
with preset tooling and do not require tool setting 
blocks. Drill jigs and boring fixtures are equipped 
with bushings for the support of the tools. 

Chip disposal is mechanized. Rotating brushes are 
provided for the cleaning of clamping surfaces as the 
part enters a fixture. A simple way of removing 
chips from operating areas and cavities within the 
parts is to provide intermediate stations where the 
part is tilted so that, the chips fall out. Other inter- 
mediate stations are used for turning the part over 

so that new surfaces are brought into position for 
subsequent machining operations. Intermediate sta- 
tions are also provided for automatic gaging of pre- 
viously machined surfaces. If a surface does not 
gage correctly, a warning signal is actuated or the 
machine is shut down. Similar warning devices are 
used to indicate if a part is incorrectly located at a 
work station. 

A typical detail from a production line is shown 
in Fig. 21-26. The picture shows two loaded work 
stations and a section of the transfer line which 
moves the part from station to station by means 
of transfer bars. The electromechanical drive seen 
in the foreground serves to raise, transfer, and lower 
the transfer bars with the parts. 

Courtesy of The Cross Co., Fastrattsfer.® 
Fig. 21-26. A transfer line with two work stations and the 
drive for the transfer bars. 




Classification of Fixtures by Grade 

The purpose of making economic estimates and 
calculations for a fixture is to justify its cost. The 
cost is affected hy the accuracy required of the fix- 
ture and particularly by the level of simplicity or 
complexity embodied in its design. This applies 
particularly to the clamping devices. In this respect, 
fixtures can be classified into four grades correspond- 
ing to four different levels of production. 

Small lot production, up to 40 pieces, requires 
fixtures of the simplest possible type with manually 
operated screw or cam actuated clamps. 

Medium lot production, from 40 to 100 pieces, 
justifies the use of quick-acting clamping devices for 
single clamping, and multiple clamping devices, where 
applicable. Multiple clamping is the simultaneous 
actuation of several clamps acting on a single part or 
the simultaneous clamping of several parts in one 

Large lot production, from 100 to 1000 pieces, 
represents the area where well designed time-saving 
clamping devices are a necessity. Multiple clamping 
is used where possible and clamps are air or hydrau- 
lically operated. 

Mass production, over 1000 pieces, uses, in prin- 
ciple, the same power operated clamping devices hut 
with the addition of such refinements as electrical 
control, remote control, or semiautomatic control 
of the clamp-actuating components. 

Estimate of Profit 

An economic estimate for a fixture shows whether 
or not it will be profitable. This involves a compari- 
son between the savings obtained by the use of the 
fixture over a fixed period and the cost of using it. 
The result depends on a number of factors. They 
can be expressed as mathematical variables and writ- 
ten into an equation, which, in turn, can be solved 

for any one of them. However, it is clearer to calcu- 
late separately each of the two items, the savings and 
the cost, and then compare them. 

The period considered is one year. The following 
symbols 1 for the variables and constants are used: 

yy* number of parts produced in a year 

s savings in labor cost per part produced in 
the fixture (dollars) 

L overhead rate (burden) on labor cost 

C cost of fixture (dollars) 

i yearly interest rate 

u yearly maintenance cost rate for the fixture 

f yearly cost of taxes, insurance, etc. 

a number of years required or estimated for 
amortization of the fixture cost. 

S (assuming an old, but still usable fixture is 
replaced by the new fixture) the unamor- 
tized value of the old fixture less its scrap 
value (dollars) 

The factors L, i, u, and t are expressed as decimal 
fractions, not as percents. 
The annual saving by using the fixture is 

X = Ns(l +£) (a) 

The annual cost Y of using the fixture is 

Y = C (i + m + t + -) (b) 


or, if the new fixture replaces an old one, the annual 

cost Y ( 

Y f = C{i + u + t+-)+Si 
' a 


'These symbols are the same as thosed used in/4 Treatise 
on Milting and Milling Machines. (Cincinnati, Ohio: The 
Cincinnati Milling Machine Co., 1951.) 3rd ed., p. 747. 




Ch. 22 

If the cost of setting up and removing the fixture 
is substantial, it is added to (b) by a term In, where 
/ is the complete cost of one setup and removal, and 
n is the number of setups in a year. 

Depending on whether 

(a) | (b)or(c) 

the fixture is profitable, breaks even, or loses money. 

Example -W\th the following values 

J = $0.10, £ = 0.90, C= $600.00, i = 0.06, 
u = 0.04, t = 0.12, a = 2 years, and S = $200.00, 
how many parts must be machined per year to 
break even? 

X = NX 10.10 X{1 +0,90)= $0.1 W (a) 

iy=$600X (0.06 + 0.04 + 0.12 + 1/2) 

+ $200 X 0.06 = $444 (c) 

hence $0.19 7V= S444 

N = $o"l9~ ~ 233? parts per year ' 

Example -'With N = 3000 parts per year and other 
values as in the previous example, the fixture is 
profitable. What is the profit? 

X = 3000 X $0.10 X(l +0.90) = $570 (a) 

Y f = $444 (c) 

Annual profit: $570 — $444 = $126 

Example-With JV= 2000 parts per year and other 
values, except C, as before, what is the maximum 
allowable cost C of the fixture? 

X = 2000 X SO.] OX (1 +0.90) = $380 (a) 

Y f = CX (0.06 + 0.04 + 0.1 2 + 1/2) 

+ $200 X 0.06 = 0.72 C+ 12 (c) 

hence 0.72 C + $1 2 = $380 

n* ■ „ $380-512 

Maximum allowable cost: C= 


= $511.11 

Example -With the following values for a fixture for 
an N/C machine tool operation 

N = 3000 parts per year; * = $0.20; L = 3.5 
(350%); C= $1600.00, and other values as before, is 
the fixture profitable, and if so, what is the profit? 

X =3000 X $0.20 X (1 +3.5) = $2700 (a) 

Y f = $1600 X (0.06 + 0.04 + 0.12 + 1/2) 

+ $200X0.06 = 51164 (c) 
Annual profit: $2700 - $1 164 = $1536 

Estimate of Fixture Cost 

The profit or loss estimate requires that the fixture 
cost is known or estimated. The safest way of getting 
this figure is to make an ordinary cost estimate from 
the drawings. However, fixture drawings may not 
be available with sufficient details for a cost estimate, 
there may not be enough time, or there may be 
other reasons why the fixture designer must make 
his own estimate of the fixture cost. 

The total cost is composed of material, labor, and 
overhead, including the design cost. The overhead 
rate is known, the material cost can be estimated 
closely enough from sketches. The labor cost can 
be estimated with sufficient accuracy for the present 
purpose from the formula 


day technology 

ff = A — — , where A = 1 05 with average present- 

H is machining and assembly time in hours, W is 
the weight of the fixture in pounds, and V is the 
overall volume of the fixture in cubic inches. V is 

calculated as length X width X height or — X 




Fig. 22-1. Overall dimensions for cost estimate. 

Ch. 22 ECONOMICS 297 

diameter 2 X height. The dimensions are measured disregarded. For the fixture shown in outline in 

over the main body of the fixture; projecting flanges Fig. 22-1 , the volume V is defined by 

are included with one third of their actual width; V= A X B X C 

local projecting parts such as bosses and feet are . ,. 

where the dimensions are taken as indicated. 

Appendix I 

Measuring Angles in Radians 

For most applications, angles are measured in de- 
grees, minutes, and seconds. However, another meas- 
uring system, "Circular Measure," is preferred for 
certain applications, e.g., when the angle is con- 
veniently defined by the length of an arc of a circle. 
Another common application of this system is in 
formulas relating to revolving bodies. The unit in 
the system is called a radian (abbreviated rad or rad.) 
and is the angle for which the arc has the same 
length as the corresponding radius. 

For a circle of radius r the length of a 1 80-degree 

arc is it X r, and it measures = n radians. 


1 80 degrees = n radians 


1 degree = tt^= 0.0175 radian, and 

1 radian = =57.2958, or 


approximately 57.3 degrees 

For small angles, the chord can be substituted for 
the arc because it is almost the same length. This 
leads to a simplification in calculations, because 
when the length of the chord is known, the angle is 
readily measured in radians, and there is no need for 
the use of trigonometric functions. 


Appendix II 

Transfer of Tolerances 

from the Conventional Dimensioning System 

to the Coordinate System 

The coordinate system of dimensioning, with the 
point of origin for the reference lines (the coordi- 
nate axes) located at or near the upper-left-hand 
corner of the workpiece, is designed to be compatible 
with the scale readings available on most jig borers. 
Many of the recently developed N/C machine tools 
are also compatible with this system. For tool and 
die work done on the jig borer, dimensions are 
usually given to .0001 inch without specified toler- 
ances, as the jig borer operator will work to the limit 
of accuracy of his machine. In this case, dimensions 
are transferred from the conventional dimensioning 
system to the coordinate system by simply adding or 
subtracting. An example is shown in Fig. 1 1 - 1 . The 
horizontal coordinate 4.7500 shown in view B is 
obtained, for instance, by the addition of the 
individual dimensions of 7/8 inch, 2 1/8 inches, 
1/2 inch, and 1 1/4 inches shown in view A. For 
other work, where tolerances are included in the 
conventional dimensions, the tolerances must be 
transferred together with the dimensions when the 
change is made to the coordinate system. The toler- 
ances in the coordinate system will not be the same 
as in the conventional system. In most cases the 
tolerances will be reduced in the transfer process. 
The new tolerances must be carefully calculated, 
otherwise serious errors will result. 

The principle of the "Transfer of Tolerances" can 
be stated as follows: 

When transferring tolerances from the conventional dimen- 
sioning system to the coordinate dimensioning system, the 
sum of the tolerances of any pair of dimensions on the co- 
ordinate system must not exceed the tolerance of the dimen- 
sion that they replace on the conventional system. 

As the first step, all dimensions with unilateral or 
with bilateral, but unequal, tolerances are changed 
to make all tolerances equal. In the second step, the 
conventional dimensions are transferred to coordi- 

REF . 





Oi Oi 

O! 0< 

o a 

oi in. 

rol KS 

Fig. 11-1 


Transfer of dimensions without tolerances. 
A, Conventional dimensions. 8, Coordinate dim- 
ensions, (Karl H. Mollrecht. Machine Shop 
Practice, vol, 1, New York: Industrial Press Inc., 

nate dimensions. In the third step, the tolerances 
are transferred to comply with the principle stated 




App. II 

The application of the principle and the three steps 
is demonstrated by the following example. In Fig. 
II- 2, views A and B show a part with conventional 
dimensioning. In view A some of the dimensions 

have mixed (unequal and equal) tolerances; in view 
B the basic dimensions are changed, as needed, to 
make a!l tolerances equal. View C shows the re- 
sulting coordinate dimensions with their tolerances. 













+i o 



Fig. II-2, Transfer of toleranced dimensions, A. Conventional dimensions with mixed tolerances. B. Conventional dim- 
ensions with equal, bilateral tolerances. C, Toleranced coordinate dimensions. (Karl H. Moltrecht. Machine Shop 
Practice, vol. 1, New York: Industrial Press Inc., 1971.) 

App. II 



Step 1. The __*__. tolerance on the 2.125 dimen- 
sion is changed to ±.002, and simultaneously, the 
2,125 is changed to 2,126. This does not change 
the physical dimension, because 


+ .003 

2.128 „ ±.002 

2.124 = 2 " 126 

Step 2. The 3.0010 dimension is obtained by the 

.8750 + 2.126 = 3.0010 

Step 3. The available tolerance on 2.126 is ±.002 
and must now be divided between two coordinate 
dimensions. If evenly divided, this leaves ±.0010 
for each dimension, resulting in .8750 -0010 and 
3.00 lO- 0010 . 

Sometimes the tolerance of a dimension on the 
coordinate-system drawing is affected by more than 
one dimensional requirement. In this case, the final 
tolerance used must be that which fulfills all of the 
requirements. For example, consider the .502— ° 02 
dimension in view B. It is replaced by the 3.0010 
and the 3.5030 dimensions in view C, If the 3.0010 
and 3.5030 dimensions are each given a ±.001 toler- 
ance, the sum of their tolerance would not exceed 
the original ±.002 tolerance and) presumably, the 
requirements for the transfer of tolerances would be 
met. However, the 3.5030 dimension together with 
the 4.6280 dimension replaces the 1.125 dimension 
in view B, which has a tolerance of only ±.001 inch. 
Thus, the sum of the tolerances on the 3,5030 and 
the 4,6280 dimensions cannot exceed ±.001 inch 
which, when divided equally, amounts to ±.0005 
inch. The 3.5030-inch dimension must therefore be 
given the lesser tolerance of ±.0005 inch. 

For this reason, when transferring the tolerance it 
is usually best to start with the smallest tolerance. 
Also note that the sum of the tolerances replacing 

the ,502 ±002 dimension is less than the ±.002 inch. 
This is satisfactory since the sum of the new toler- 
ances does not exceed the original tolerances. 

When a tolerance is bound by a small tolerance, as 
in the case of the .502 dimension, it is sometimes 
possible to increase an adjacent tolerance on the 

coordinate dimension drawing. For example, the 
2.000-inch dimension in view B, which has a toler- 
ance of ±.005 inch is replaced by a 1.0000- and a 
3.0000-inch dimension on the coordinate drawing. 
Each of these dimensions could be given a tolerance 
of ±.0025 inch; however, the tolerance of the 1 .0000- 
inch dimension on the coordinate drawing is bound 
by the requirement of the ±.001 tolerance of the 
1.000 dimension shown at the right side, in view B, 
Thus, the 1 .0000 dimension in view C, together with 
the 2,0000 dimension, must have a total dimension 
tolerance of ±.0010 or ±.0005 inch on the 1.0000- 
inch dimension. Since the sum of the tolerances of 
the 1,0000 and the 3.0000 dimensions (view C) can 
be ±.005 inch, the tolerance of the 3.000 dimension 
can be increased to ±.0045 inch. 

The basis for the determination of the final toler- 
ances for the coordinate dimensions (view C) is 
summarized below: 

8 7SQ±J>010 : jhe ±.0010 tolerance together with 
the ±.0010 tolerance of the 3.0010 
dimension is required to maintain 
the ±.002 tolerance for the 2.126 

3.0010 ±001 °: See requirements for the 

g 7 50±.ooiO dimension given above. 

3.5030 ±OOOS : The ±.0005 tolerance together with 
the ±.0005 tolerance of the 4.6280 
dimension is required to maintain 
the ±.001 tolerance for the 
1.125+.- 001 dimension. 

4.6280 ±OOOS : See requirements for the 

3,5030 ±OOOS dimensions given 

1.0000 ±0005 : The +.0005 tolerance together with 
the ±.0005 tolerance of the 2.000 
dimension is required to maintain 
the +.001 for the 1.000 ±00) dimen- 
sion given at the right, in view B. 

2.0000 ±ooos : See requirements for the 

1.0000 ±00 ° 5 dimension given 

3.0000*' 0045 : The .0045 tolerance, together with 
the ±.0005 tolerance of the 1,0000 
dimension in view C, is required to 
maintain the ±.005 tolerance for the 
2.000 ±005 dimension in view B. 

Appendix III 

The Dimensioning of Fixtures by Stress Analysis 

The Dimensioning of Fixtures by Stress Analysis 

Although the structural design of fixtures has not 
been given much consideration in most textbooks 
on stress analysis, they can be designed systemati- 
cally by the proper application of known formulas 
and calculation procedures. An underdimensioned 
fixture may be damaged or destroyed in use. An 
overdimensioned and, therefore, overweight fixture 
is a constant source of unnecessary expense for ex- 
cessive work in handling, transportation, and storage, 
etc., of the fixture. The forces for which the fixture 
is analysed are the external loads, the clamping loads, 
and the reactions. The external loads comprise the 
cutting forces, the weight of the part and the fixture, 
and inertia forces. Inertia forces are the centrifugal 
forces in lathe fixtures and rotating grinding fixtures, 
and the deceleration and acceleration forces at stroke 
reversal in fixtures for planers and surface grinders . 

A nil e-of- thumb says that a fixture stress analysis 
shall be performed when the weight of the part is 
25 pounds (1 ION) or more. This weight is exempli- 
fied by a 3- by 3- by 10-inch (75- by 75- by 250-mm) 
solid block of steel, or by a hollow aluminum casting, 
open on one side, with 1/2 inch (13 mm) wall thick- 
ness and 8- by 8- by 16-inch (200- by 200- by 
400-mm) overall dimensions. The cutting forces run 
into hundreds, if not thousands of pounds, and are 
always somewhat approximate. There is, therefore, 
no need to include the weight of the part and the 
fixture in a static stress analysis as long as these 
weights do not exceed 10 percent of the main 
cutting force. 

Formulas for calculating centrifugal forces are 
found in the Mechanics sections of reference books, 
such as Machinery's Handbook. 1 The acceleration 

of a planer table at stroke reversal is of the order of 
magnitude of from 0.0 lg to OAg and is insignificant 
except in special and extreme cases. 

To calculate the load from the cutting tool, it is 
resolved into its three components as shown in Fig. 
III-l. They are: 

Eg, The main cutting force or, simply, the cutting 
force. It is the force component acting in the direc- 
tion of the tool travel (the direction of cut) relative 
to the workpiece, [n a cylindrical turning operation 
it is the tangential force component. 

Fp, The feed force. This is the force component 
acting in the direction of the feed, i.e., parallel to 
the surface which is being generated in the machining 
operation. In a cylindrical turning operation it is the 
longitudinal force component, 

Ff, The thrust force. This is the force component 
which acts in the direction perpendicular to the sur- 
face being generated. In a cylindrical turning opera- 
tion it is the radial force component. 

Force components are in pounds or newtons. A 
single-point tool has only one set of force compo- 
nents. For multiple-point tools (drills, milling cut- 
ters, broaches) there is a set of force components 
for each cutting edge which is actively cutting. Fq 

Eric Oberg and F, D. Jones, Machinery's Handbook 
(New York: Industrial Press Inc., 197 T) 19th ed,, pp. 

Fig. Hl-l. Three force components of the cutting tool. 


App. Ill 



is the major force component and is the component 
that determines the amount of work and horsepower 
absorbed in the cutting operation. Fp and F T are 
significantly smaller than F c . Average values are 

1 2 

F F « — F c to — F c , and 

F T *^F c io-F c 

Fp is maximum and F T is minimum when the side- 
cutting-edge angle (SCEA) is zero;Fp decreases and 
Fj increases with increasing SCEA . The size of F^ 
and the other force components depends on the ma- 
terial, the dimensions of the cut, and the cutting 
speed. Detailed data are found in reference and text 
books. However, for the purpose of dimensioning 
fixtures it is sufficient to use the approximation that 
F c equals the unit (specific) cutting pressure p c 
multiplied by the area of cut A Q : 

F c = p c A Q =p c fd 


A = area of cut (square inches or mm 2 ) 

/ = feed per revolution or per tooth (inches 
or mm) 

d = depth of cut (inches or mm) 

p c is essentially a material constant and can be 
taken as 2.5 to 3.2 times the tensile strength for 
steel and other ductile materials, and 

4.5 to 5.6 times the tensile strength for cast iron 
and other brittle materials, 

where the effects of dimensions of cut and cutting 
speed are reflected in the ranges quoted for the co- 

efficients. The higher values are to be used for fine 
feeds or shallow depths of cut (small / and d) and 
lower cutting speeds (as used with high-speed steel 
tools), the lower values are for heavy cuts and/or 
higher cutting speeds (as used with carbide and 
ceramic too! materials). In the final calculation of 
Fp and Ff a contingency factor is introduced to 
allow for tool wear, cutter runout, and local varia- 
tions in material dimension and hardness. For single- 
point tools and drills, this factor is 1.25. For milling 
cutters it is 2. For twist drills, Fp is further in- 
creased by a factor of 1 .33 to allow for the additional 
resistance caused by the chisel edge. Data for drilling 
forces are found in text and reference books. 2 ' 

The clamping forces must secure the part against 
being pulled out of the fixture by the cutting forces. 
Detailed calculations for the various types of clamps 
are given in Chapter 10. The safety factor against 
pullout should be not less than 1.5, however, in most 
cases it will be found that a safety factor of 2 or 
better can easily be established. 

With the forces calculated, the elements of the 
fixture can now be dimensioned. Regardless of how 
complicated the fixture may appear, with a little 
practice on the part of the designer it can always be 
subdivided into simple structural elements. These 
elements are cantilever beams, simple beams, shafts 
and bolts (loaded in torsion and/or bending), flat or 
curved plates of square, rectangular, or circular cir- 
cumference, cylinders, angles, and, occasionally, col- 
umns. Formulas for dimensioning these are found 
in Machinery 's Handbook. 

2 Ibid.,pp. 1743, 1744. 

3 Karl H. Moltrecht, Machine Shop Practice (New York: 

Industrial Press Inc., 1971) vol. 1, p. 76. 

"Oberg.op. eif.,pp 402-441. 

Appendix IV 

Metric Conversion Tables 

Fractional Inch- — Millimeter and Foot — Millimeter Conversion Tables 
(Based on I inch ™ 25.4 mil I i meters b exactly) 




In. Mm. 

In. Mm. 





mt 6.747 

a ?6t 13*97 





)i» 7-144 

1^2 13.494 





1;** 7,541 

*$i* 13.891 


20. 241 



H« 7.938 

ff* 14.288 





Hit 8. 334 

m* 14.684 





'Hz 8.731 

'ISa 15.081 


21 431 



'Hi 9.128 

*Hi IS. 478 





H 9 525 

*E 15.87s 





2J|l 9-922 

* lit 16.272 





1453 io.3r9 

»H» 16.669 




4 366 

sjii 10.716 

<H* 17.066 





lAi 11. 112 

'Ha 17.462 





2)4* 11 . 509 

•Hi 17.859 





«Ma 11.906 

»Hj ts. 356 




5 953 

3 iii 12.303 

*J4« 18.653 




6. 350 

ii 12 700 

H 19.050 




In. Mm. 

In. Mm. 

In. Mm, 

In. Mm. In. Mm. 





3 76.2 

5 127.0 

1 177.8 9 328.6 




So. 8 

4 10I.6 

6 1S2.4 

8 203,2 10 254.0 






Ft. Mm. 

Ft, Mm. 

Ft, Mm. 





10 3.048 

1 304. 8 

0.1 30.48 





20 6,096 

2 609.6 

0.2 60.96 





30 9,144 

3 914-4 

0.3 91.44 





40 52.192 

4 1,219,2 

0.4 121.92 





50 15,240 

5 1,524.0 

0.5 IS2.40 


IS, 240 


60 1 8.288 

6 1,828.8 

0.6 182.88 





70 31,336 

7 2,133,6 

0.7 213.36 





80 24,384 

8 2,438.4 

o.S 343.84 





90 27.432 

9 2,743.2 

0.9 274.32 





100 30.480 

10 3,048.0 

1.0 304.80 



Example i: Find millimeter equivalent o£ 293 feet. $*%* inches. 

200 ft — 60,960, mm 

90 ft - 17.432. mm 

3 ft - 914 4 mm 

5 in. ■ 127.0 mm 

*%< in. — 18.653 mm 

393 ft, S*JtU in- ■ 89,452.053 nun 

Example 2: Find millimeter equivalent of 71.86 feet. 

70, ft — 21,336, mm 

1. ft = 304,8 mm 

.8© ft = 243.84 mm 


ft- 18.288 mm 

71. 86 

ft ■ 21,902.928 mm 


App. IV 





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Acme thread, 125 
Acorn nuts, 116,195,203-204 
Actuators, plunger, 216 
Adapters, 281-283 

for vises, 129-130,235-236,272 
Adhesive clamping, 133 
Adhesive for bushing installation, 158- 

Air blast for chip removal, 83-84 
Aircraft tooling, 275-277 
Air cushion table, 228 
Air cylinders, 288-290 
Air powered clamps, 1 IS, 147, 241, 

269, 282, 288-290 
A1SI steels, 20-21 
Aligning clamps, 171-172 
Alignment of fixture to machine table, 

17 1-172,247,252 
Allowance, machining, 22, 32-33, 183 
Alloys, lead-tin-antimony, 44 

zinc base, 44 
Alloy steels, 21 

Aluminum inserts for lathe chucks, 208 
Aluminum tooling plate, 2 3, 168, 183 
American Standard Jig Bushings, 155 
Angle block, 234 
Angle plate, 287 
Angle plate grinding fixture, 270 
Angle, side cutting-edge (SCEA), 303 
Angles in radians, measuring (App.), 

Angle strap, 113, 120 
Angular clamps, 120-123 
Angular drilling, 233-2 36, 238 
Angular errors (misalignment), 27-28, 

54, 55, 91,93 
Angular indexing, 58-61 
Anneal, of castings, 179 

ofweldments, 182 
Arbors and mandrels, elastomers for 

expanding, 263-264 
Arbors, hydraulic, 263, 291 

materials for, 22 
Arc blow in welding, 278 
Archimedes spiral, 110 
Arc welding fixtures, 275-280 
Assemblies, clamp, 205 
Assembly fixtures, 275 
Assembly screws, 174 

materials for, 22 
Automatic centralizers, 97-101 
Automatic end cam clamp assemblies, 

Automatic fixtures, 287-2 91 

Backing bars, 278 
Backlash, 97, 102-104 
Baffle plates for chip and coolant con- 
trol, 83-84 
Ball bearings, 59 

Ball knobs, plastic and steel. 201-202 
Ball plunger, 217 
Ball, tooling, 217 
Bar knobs, 201 

Base for a jig, separate, 234-235 
Bases fixture, 170 
materials for, 23 
Bayonet-lock clamp, 128, 238 
Bayonet type guide groove, 205 
Bayonet type locking device for slip 

bushings, 161-162 
Bayonet type T-bolt, 205 
Beam clamps, 113-115 

Beams, 61, 113-115 
Bearings, ball and roller, 59 

outboard, 60 
Bell-crank, 120 
Bell-crank levers, 98-99 
Bevel gears, 103 

Blocking for foolproofing, 71, 73, 75 
Blocks, dummy 285 
inertia, 260 
step, 207-208 

tool setting, 2, 151,244,292 
Blueprints and specifications for the 

part, ^ 
Bodies, cast, 44-45 
jig or fixture, 20, 170-184 
welded, 45 
Bolts, eye, 195 
hook, 1 1 6 
index, 59 
jig latch, 195 
materials for, 22-23 
T-, 171, 195 
Bonding agent, 195 

American LoctitetfiJ 1 S3 
Booster, pneumatic-hydraulic, 290-291 
Boring bars, 265, 268 
materials for, 22 
stub, 26S, 268, 269 
Boring fixtures, 265-269 
Boring machines, production, 266, 269 
Boring mill, horizontal (HBM), 265-266 
Boring positioner, 208 
Borizing, 266 
Box-type drill jigs, 138, 170, 173, 224, 

227,232-233, 238 
Bracket type jigs, 125,228,238 
Broaching fixtures, 272-275 
Bronze locators, 45 
Buckling instability, 141-142 
Built-up fixtures, 172, 173, 175-176, 

Bullet-nosed pins, 2 1 
Burrs, 5, 40, 76-77, 82-83, 95, 230 

relief for, 84-86 
Bushing holder for slip bushings, 238 
Bushing plate, 235 

Bushings, American Standard Jig, 155 
ceramic coating on 168 
circuit board, 169 
close together, 158, 163, 229 
conventional, 154 
crimping of, 169 
distance from ends of, 229-230 
drill, 4, 154-169 

ferrous titanium carbide for, 167 
fixed, 154, 155 
floating, 9S-97 

formulas for designing, 165-167 
guide, 1 64 
headless, 154, 155 
head type, 154, 155, 15 8, 159 
liner, 154, 155, 156, 158, 159 
locking clamps for renewable, 161-162 
long, 165 
loose, 154, 155 
master, 155 

materials for, 22-23, 167-168 
non-standard, 1S4, 163- 1 64, 165-167 
potted, 168, 184 
portable power tool, 164-165 
press-fit, 155, 156, 158 
renewable (loose), 154-156, 159-163 
retainer, 208 

Bushings, screw, 95-97 
slip, 155, 159-163 
special, 154 
standard, 154-156 
stationary press-fit, 154 
template, 168-169 
thin-wall, 163 
threaded, 9S-97, 163-164, 229, 233 

USA Standard, 154-156 
wide, 165 
Buttons, 46-48 

design rules for, 48 

hollow, 47, 208 

installation of, 46-47 

materials for, 46 

rest, 48, 208 

spherical, 208 

stop, 46 

threaded, 47, 201, 208 

used as locators, 46, 201, 208-209 

Cables, connecting 196, 2 14 
Callouts, 191-193 
Cam clamp assemblies, 205-207 
Cam clamps, 108-111,205-206 
Cams, 97, 99-100, 108-1 11, 125-129, 
205-207, 290 
eccentric, 110-111, 125-128 
materials for, 22-23 
self-locking, 106, 109-111 
spiral, 110 
Camshaft grinding fixture, 27 1-272 
Captive locating pins, 214-215, 22 S 
Carbides, sintered (cemented), 23, 

J67, 268 
Case series, 219-227, 245-252, 266- 

267, 279-280 
Castable materials for nesting, 44 
Castable tooling, 21 1, 2 13 
Cast fixtures, 172, 173, 176-179, 183 
bodies for, 44-45 

rules for dimensioning of, 1 77-179 
Castings, accuracy of, 35 
broken edges in, S, 82 
draft on, 34-35 
effect of machining on, 179 
machining allowances for, 32-33 
minor irregularities in, 35, 95 
mismatch in, 35 
normalizing of, 179 
shrinkage and warpage of, 33 
stabilizing treatment of, 179 
steel, 23 

tolerances for, 33-35 
types and applications of, 23 
uniformity of, 35 
Cast iron, ductile, 23 
gray, 23 
nodular, 23 
Cast iron fixture stock sections, 217 
Cemented (sintered) carbides, 23, 167, 

Center drill, 94 

Centering, single, double, and full, B7 
Centralizers, 87-104, 232 
automatic, 97-101 
for gear wheels, 101-104 
linkage controlled, 89, 97-101 
Centrifugal forces, 259, 302 
Cerrobend®, 44, 134 
Chains, kinematic, 90, 97, 287, 289 
Channel fixture, 173 
Channel jig, 227 




Chip breakers, 82 

Chip deflector, 84 

Chip disposal, 83-84 

Chip removal, 83-84, 247, 284 

Chips, drilling, 82, 229-2 30 

milling, 82 
Chip types, 82 
Chip volume. 82 

Chuck fixtures for lathes, 259-260 
Chuck into a turning fixture, converting 

a, 208 
Chuck jaws, 207-208 

materials for, 22 
Chucks, collet, 87, 90-91, 260 

drill, 90 

electrostatic, 132-133, 269 

inserts for lathe, 208 

lathe, 208 

magnetic, 130-132, 269 

self-centering, 87, 89, 90, 102 
Chuck type grinding fixtures, 102-103, 

Chutes for chip removal, 83-84 
Circuit hoards, bushings for, 169 

drilling of, 241-242 
Circular locators, 51-58, 254 
Clamp assemblies, removable, 205 
Clamping, 19 

adhesive, 133 

economics of, 1 1 5, 288 

elements, 105-135,207-208 

fixture to machine tool table, 171 

lathe fixture with floating action, 

multiple, 144-145 

of a package, 141-142 

of gas turbine compressor blades, 134-135 

of honeycomb with ice, 135 

vacuum, 133, 269 

with low-melting alloys, 134-135, 147 
Clamping devices, operating time for 

(Table), 1 1 S 
Clamping forces, 303 

Wm screws (Table), 108 
Clamping screws, 107, 108, 115-1 17, 

Clamping wedges, 105-106, 122, 147- 

dual action, 64-67 
Cla mps, 105 

air pUWeted .115 

angular, 120-123 

beam, 113-115 

cam, 108-11 1 

double movement, 121, 139-140 

dual pressure, 289, 290 

for renewable bushings, 161-162 

hydraulically powered, 1 1 S 

materials for, 22, 23 

quick-acting, 127-1 28,-2 39, 241 , 281 

safety feature for air-operated, 242, 
244, 288-289 

Strap s for, 207-208 

/Cfglerrn-l 13, 125. 128-129, 136, 
* 216-217,243, 279 
Class II copper for backing bars, 278 
Clearance in fixtures, 68, 76, 172, 227 
Closed jig, 170, 224-227 
Cloth, glass fiber, 24, 40-41, 44 
Clutch, overrunning, 282 
Coating on bushings, ceramic, 168 
Cold finished material, 22 
Collet chucks, 87, 90-91 
Collets, materials for, 22 
Color code, 170 

Components, commercially available, 4, 

indexing, 215-217 

locating, 42-67, 208-209 

patented, 195 

propietary, 195 

wear on indexing, 58-60 
Conical locators, 93-97 
Connecting cables, 196-214 
Construction ball, toolmaker's, 2 I 7 

Contour grinding, 272 

Contour milling fixtures, 252-253 

Conventional bushings, 154 

Conversion tables, metric 
(App.), 304-306 

Converting a chuck into a turning fix- 
ture, 208 

Conveyor, 57 

Coolant control, 83-84 

Coordinate dimensioning system, 186- 
189 (App.), 299-301 

Cost of fixtures, 183, 296-297 

Coulomb's Law, 133 

Coupling nuts, 195 

Cradle, 227, 234, 272 

Crank arms, materials for, 2 3 

Crankpin grinding fixture, 27 1 

Crankshaft fixtures, 265 

Crimping of bushings, 169 

"Cross" and crossed diamond pins, S6-57 

Cup locators, 95 

Cutter guidance, 20 

Cutter guides, 151-153, 246, 249, 251, 

Cutting forces, 272-273, 302-303 

C-washers, 120, 196 
swing, 197 

Cylinder location, dual, 29, 54-58, 293 

Cylinders, power, 287 

Cylindrical locators, 28-30 

Damping vibrations, 23 

Defects in die forgings, minor, 38 

Deflector, chip, 84 

Deformation analysis of fixtures, stress 

and (App.), 302-303 
Degrees of freedom, 18,26-28, 61,251 
Depth machining, 6 
Design, of bushings, 16S-167 

of drill jigs, 219-243 

of fixture bodies, 18-24, 170-185 

of fixtures (App.), 302-303 
Design rules, for buttons, 48 

for welding, 181-182 
Detent pins, 214 

Determinate system, statically, 61, 147 
Diamond pins, 56-57, 210, 254-256 
Die forgings, draft on, 38 

flash extension on, 36-38 

minor defects in, 38 

mismatch in, 36-38 

tolerances for (Table), 36-38 
Dielectric, 133 
Dimensioning of cast fixtures, rules 

for, 177-179 
Dimensioning of drawings, 186-189 
Dimensioning systems, coordinate, 1 86- 

189, (App.) 299-301 
Dirt, relief for, 84-86 

seals and shields for, 86, 210 
Disposal of chips, 83-84 
Distortion in weldments, 38, 275-278 
Dividing head, 59, 2S4 
Dogs, gripping, 120-121, 1 50, 152, 160 
Double, (def.) 25 
Double-action pins, 214 
Double centering, 87 
Double locking levers, 202 
Double movement clamps, 121, 139- 

Double nested, 25 
Double or triple screw thread, 135 
Dowel pins, application of, 50-51, 
173, 174 

materials for, 22, 174 
Draft, on castings, 34-35 

on die forgings, 38 

on molded plastic parts, 40 
Drawings, 170, 185-186 

dimensioning of, 186-189 
Dressers, grinding-wheel, l 52, 270 
Drill bushings, 4, 154-169 

floating, 95-97 

materials for, 22-23, 167-168 

threaded, 95-97, 163-164, 229, 233 

Drill, center, 94 
Drill chuck, 90 
Drilling chips, 82, 229-230 
Drilling of circuit boards, 241-242 
Drilling without bushings, 242 
Drill jigs, (def.) 1,97-101, 125, 138, 
170-171, 219-243, see also Jigs 

design of, 219-243 

design procedure for, 219 

for half holes, 239-240 

for heavy work, 235 

indexing, 60-61, 227, 228, 236-238 

operating with, 227-229 

universal, 170, 281-284 
Drill press, fixed-spindle, 227-228 
Drill, radial, 227-228, 268 
Drop forgings, machining allowance on, 

Dual action clamping wedge, 64-67 
Dual cylinder location, 29, 54-58, 293 
Dual fixtures, 292-293 
Dual fixturing for N/C machines, 258 
Dual pressure clamps, 289, 290 
Ductile cast iron, 23 
Ductliron®, 23 
Dummy blocks, 285 
Duplex milling, 245, 252, 253 

F.ccenters, materials for, 2 3 

Eccentric cams, 1 10, (Table) 111, 125- 

Eccentricity, 91, 110-111 
Eccentric leveling lugs, 2 10 
Economy of fixtures, 2, 10, 185, 242, 

288, 295297 
Ejectors, 77-80, 217, 277 
Elastomers for expanding mandrels and 

arbors, 263-264 
Electromagnets, 81, 131 
Electrostatic chucks, 132-133, 269 
Engagement for jam-free locators, 

length of, 5 1 
Epoxies, 23-24, 44, 279 
Equalizers, 90, 124, 136-146, 149-150, 

hydraulic, 145-146 
plate-type, 138 
Errors, angular (misalignment), 27-28, 

54, 55,91 
in part surface geometry, 27 
Estimate, of fixture cost, 296-297 

of profit, 295-296 
Expanding mandrels and arbors, 

elastomers for, 263-264 
Eye bolts, 195 

Face milting cutter, 251 

Face plate fixture, 64-67, 95, 260-262 

Face plate, magnetic, 81, 130 

Fatigue, operator, 107-108 

Feeler gage, 152-153, 246,249 

Feet, jig, 171.209,221, 227,233,234 

FERRO-T1C®, 2 3 

Ferrous titanium carbide for bushings, 

Field shapers, 132 
Finger handle, 202 

Fire hazard in machining magnesium, 23 
Fixed bushings, 154, 155 
Fixed locating components, 208-209 
Fixed-spindle drill press, 227-228 
Fixture bases, 170 

materials for, 23 
Fixture bodies, 20, 172-1 84 

Cast, 44, 45 

design of, 170-184 

materials for, 2 3 

one-piece (solid), 175, 183-184,238 

welded, 45, 172, 173, 179-183 
Fixture clamping to machine tool table, 

Fixture components, commercial and 

standard, 4, 194-218 
Fixture cost estimate, 296-297 
Fixture design, check list for, 10-17 



Fixture design, general considera- 
tions in, 9 
preplanning of, 5-8 
procedure, 18-24, 185 
Fixture materials, 20-24 
Fixtures, (def.) 1 
broaching, 272, 273-275 
camshaft grinding, 27 1-272 
cast, 172, 173, 176-179, 183 
channel, 173 
classification of, 295 
clearance in, 68, 76, 172, 227 
cost of, 183,296-297 
crankpin grinding, 271 
crankshaft, 265 
dual, 292-293 
economy of, 2, 10, 185, 242, 288, 

estimating cost of, 296-297 
faceplate, 64-67, 95, 260-262 
foolproofing, 69, 71-76 
gear nobbing, 256-257 
grinding, 102-103, 269 
hydraulic, 287-288, 290-291, 292- 

indexing, 58-61, 86, 254, 273-274, 

lathe, 149, 259-265 
materials for, 20-24 
milting, 2, 244-258 
NjC machine tool, I, 187, 257-2 5 8, 

259, 291-293 
pallet, 58,293-294 
patented, 195 
planing, 273 
pneumatic, 287-290 
proprietary, 195 
radial, 254 

rotating, 254-255, 2 56-2 57 
rules for dimensioning of cast, 177- 

shaping, 272 
slotting, 272-273 
space in, 68, 76, 172, 227 
structural design of (App.), 302-303 
transfer, 58, 292-294 
universal, (def.) 2, (def.) 281, 284- 

welding, 275-280 
Fixture stock sections, cast iron, 217 
Flange nuts, 195, 205 
Flash extension on die forgings, 36-38 
Floating action clamping, lathe fixture 

with, 67, 95 
Floating drill bushings, 95-97 
Floating pin locators, 212-213 
Floating principle, 67, 95, I 36-140, 

Floating screw, 139 
Floating zero, 292 
Flux, magnetic, 132 
Foam, polyurethane, 24 
Foolproofing, embracing fork for, 73-7 5 
punched parts, 76 
the fixture, 69, 71-76 
Force from screws, clamping (Table), 

Force on magnetic chucks, holding 

(Table), 131 
Forces, clamping, 303 
cutting, 272-273, 302-303 
inertia, 260, 272, 302 
manual, 107-108 
Fnrgings, see also Die forcings. Drop 
draft on die, 38 
flash extension on, 36-38 
machining allowance on, 33 
minor defects in, 38 
mismatch in, 36-38 
shrinkage tolerances for, 37 
tolerances for, 35-38 
45-degree plungers, 137, 139 
4-2-1 locating principle, 27 

Freedom, degrees of, 18, 26-28, 61, 

Friction, coefficients of (Table), 106 
Fulcrum pins', materials for, 23 
Full centering 87 
Fully, (def.) 25 
Fully nested, 25 

Gages, feeler, 152-153, 246, 249 

setting, 151,253 

set-up, 1 51 
Gang milling, 152, 191, 245, 248 
Gear hohbing fixtures, 2 56-2 57 
Gears, 102-104 

Gear wheels, centralists for, 101-104 
Glass fibers, 24 

cloth of, 24, 40-41, 44 

mats of, 24 

roving s of, 24 
Gray cast iron, properties of, 23 
Grinding, contour, 272 
Grinding fixtures, 102-103, 269-272 
Grinding-wheel dressers, 152, 270 
Gripping dogs, 1 20-12 L, 150, 152, 160 
Guidance, cutter, 20 
Guide bushings, 164 
Guides, cutter, 151-153,246,249 
251, 252 

Half holes, drill jig for. 239-240 
Half-turn screws, 201 
Hand knobs, 116-117, 199, 203-205, 

four- and five-pronged, 20 1 

materials for, 23 

star, 201 
Hand- knob screws, 147, 199,251 
Handle, finger, 202 

speed, 205 
Handles, machine, 202 

speed-ball, 201 
Handwheels, 202 
Hardened washers, 23 
Headless bushings, 154, 155 
Head type bushings, 1 54, 155, 158, 159 
Heat treated parts, tolerances for, 40 
Heavy work, drill jigs for, 235 

loading, SO 
Helical gears, 102-104 
Helix angle in gears, 103 
Hinge clamp assemblies, 2 05 
Hobbing fixtures, gear, 256-257 
Hold-down bolts, lugs for, 179 
Holding fixture, welding, 278 
Holding force on magnetic chucks 

(Table), 131 
Holes, tooling, 57, 170,252,293 

tooling for tapered, 267-268 
Hollow buttons, 47, 2 08 
Hook bolts, 116 
Hook clamp assemblies, 205 
Horizontal boring mill (HUM), 265-266 
Horn for a broaching fixture, 274-275 
Hydraulically powered clamps, 1 15, 

145-146, 147,292-293 
Hydraulic arbor, 263, 291 
Hydraulic equalizers, 145-146, 190 
Hydraulic fixtures, 287-288, 290-291, 

Hydraulic mandrel, 263, 291 

Index bolt, 59 
Indexing, angular, 58-61 

rapid, 60 

straight line, 58 
Indexing components, 215-217 

wear on, 58-60 
Indexing drill jigs, 60-61, 227, 228, 

Indexing fixtures, 58-61, 86, 254, 273- 

Indexing plungers, 216-217 
Indexing table, 58-60 
Inertia block, 260 
Inertia forces, 260, 272, 302 

Inserts for lathe Chucks, 208 
Instability, buckling, 141-142 
Integral locators, 44-45 
Integral locking tabs for slip bushings, 

Integrity, structural, 2 
Intensifier, pressure, 290 
Intermediate supports, 147-150, 209- 

Internal broaching, fixtures for, 274 
Irregularities in castings, minor, 35, 95 

Jack locks, 209 
Jacks, 147 

spring loaded, 246 
Jack screws, 117-118, 201,224 
Jam-free locators, 51-53 
Jamming, 51-53 
Jaws, chuck, 207-208 

detachable vise, 130 

materials for chuck and vise jaws, 22 
Jig base, 234 

Jig borer, 158, 186, 191, 266,299 
Jig bushings, placement of, 229-231 
Jig feet, 171, 209,221, 227,233, 234 
Jig grinder, 158, 186, 270 
Jig latch bolts, 123-124, 195 
Jig legs, 209,221,222, 223,226, 227 
Jig or fixture body, 20 
Jig plate, 2 1 9-2 2 1 , 224, 22 5 
Jigs, (def.) I, see also Drill jigs 

chanoel, 227 

closed, 170, 224-227 

custom-made, 281 

for large work, 235 

leaf, 124, 170, 226, 227, 232 

open, 170, 221-224, 227, 231-232 

plate, 170, 219-221, 227, 231-232 

pump, 170,240, 241, 281 

reversible, 231 

rockers for, 235 

template, 227 

tumbling, 227, 228, 283, 284 

universal, 208 
Jigs and fixtures, origin of, t 
Jig stand, 234 
Joints, welded, 179-180 

Keys, and keyseats for radial locating, 

for aligning fixture to machine table, 

sine fixture, 195 
Kinematic chains, 90, 97, 287, 289 
Kirksite® 44 

Knobs, hand, 116-117, 199,203-205, 

knurled, 201 

materials for hand, 23 

quarter-turn, 125 

quick-locking, 202 

steel ball, 201-202 

star hand, 201 
Knob swivel screws, 1 99-200, 205 
Knurled head screws, 200 
Knurled knobs, 201 
Knurled lock nuts, 195 

Laminated plastic sheets, 24 

Laminates in plastic tooling, 23-24 

Laminating resins, 24 

Large work, jigs for, 235 

Latch, 138 

Latch bolt, jig, 123-124, 195 

Lathe chucks, 208 
inserts for, 208 

Lathe fixtures, 149,259-265 
pot type, 172, 262 
with floating action clamping, 67, 95 
with recentering device, 64-67 
with size adjustment, 64-67 

Lathe mandrel, 94 

Lathes, chuck fixtures for, 259-260 
welding, 280 



Leaf, 115, 123-125,205,226 

swinging, 115, 123-125, 205, 226 
Leaf jig, 124, 170, 226, 227, 232 
Legs, jig, 209,221, 222, 223, 226, 227 
Leveling lugs, eccentric, 210 
Levers, locking, 202 
Line boring bars, 265, 268 
Line boring fixture, 266-2 67 
Liner bushings, 154, 155, 156, 158, 

Linkage controlled centralizers, 89, 97- 

Linkages, pantograph systems, 97, 100- 

scissor type, 97 
"Liquid steel," "liquid aluminum," 24 
Loading and unloading the part, 68-81 
Loading heavy parts, 80 
Locating buttons, threaded adjustable, 

Locating by sighting, 42-43 
Locating components, 42-67, 208-209 
Locating error, radial, 91 
Locating pads (rest pads), 48-5 1, 208 
Locating parts (locators), materials for, 

Locating pins, 48, 210-215, 221, 222, 

captive, 214-215, 225 
Locating plane, disadvantage of inclined, 

Locating, preparations for, 32-4 1 

with keys and keyseats, radial, 54 
Locating principle 4-2-1, 27 
Locating principles, 2 5-31 
Locating the part, 1 8, 68, 87 
Locating unmachined surfaces, 32 
Location, dual cylinder, 29, 54-58, 293 
Locator buttons, 46, 201, 208-209 
Locator pins, 48, 210-215, 22!, 222, 

Locators, 42-67, 208-209, 281-282 

adjustable, 61-67, 209-210 

circular, 51-58, 254 

conical, 93-97 

cup, 95 

cylindrical, 28-30 

floating pin, 212-213 

integral, 44-45 

jam-free, 51-53 

materials for, 22, 45-46 

radial, 29, 54 

rotational, 29 

screw tvpe adjustable, 61-67, 209-210 

sliding, 94-95 

sliding point type adjustable, 63 

slotted hole, 213 

spherical, 52,90, 102,208 

Split-cylinder type, 63-64 

unhardened, 46 

wear on, 45-46 

wedge type adjustable, 62-63 

with triangular relief, 52-53 
Locking clamps for renewable bushings, 

Locking devices for slip bushings, 160 
Locking levers, 202 
Locking mechanism for universal jigs, 

Locking tabs for slip bushings.integral 

Lock nuts, knurled, 195 
Locks, jack, 209 
Loctite®, American, 158 
Long travel cam clamps, 205-206 
Loose bushings, 154, 155 
L-pins, 213-215 
Lugs, eccentric leveling, 2 1 

for hold-down bolts, 179 

Machined parts, tolerances for, 39-40 
Machine handles, 202 
Machines, transfer, 57-58, 266, 

Machine table, keys for aligning fix- 
ture to, 171-172,247, 252 
Machine tool bed, boring fixture for, 

Machine tool table, clamping fixture 

to, 171 
Machine tool vises, 90, 244, 252, 281 
Machining allowances, 22, 32-33, 183 

for castings, 32-33 

for drop forgings, 33 

for forgings, 33 
Machining, effect on castings, 179 

depth, 6 

surface, 6 
Machining fixtures, classification of, 2 
Machining magnesium, fire hazard in, 23 
Machining parameters, 5 
Machining stresses, 179 
Magnesium machining, fire hazard in, 

Magnesium tooling plate, 2 3, 168, 183 
Magnetic chucks, 81, 130-132, 269 

holding force on (Table), 1 31 
Magnetic faceplate, 81, 130 
Magnetic flux, 132 

in welding, 278 
Magnets, permanent, 81, 131 
Major burr, 82-83 
Malleable iron, 23 
Mandrels, 125, 263-265 

gear hobbing, 256 

hydraulic, 263, 291 

lathe, 94 

materials for, 22 

threaded, 264-265 
Mandrels and arbors, elastomers for 

expanding, 263-264 
Mandrel type fixtures, 263-265 
Manipulation and operator .criteria, 9 
Manual forces, 107-108 
Manual work fixtures, classification 

of, 2 
Manufacturing operations plan, 5 
MarKs, target, 58 
Masonite®, 168 
Master bushings, 155 
Master tooling for aircraft industry, 27S 
Materia! selection, steel, 21-2 3, 167 
Mats, glass fiber, 24 

Measuring angles in radians (App,), 298 
Mechanisms, 97 
Meehanite®, 23 
Metric conversion tables (App.), 304- 

Milling chips, 82 
Milling cutter, face, 25 ] 
Milling fixtures, 2,244-258 
Milling, duplex, 245, 252, 253 

gang, 152, 191,245,248 

multiple, 245 

run-in and run-out distances in, 244 

string, 141, 244-245, 252-253 

tracer, 255 
Milling machine dividing head, 59, 254 
Misalignment (angular errors), 27-28, 

54, 55,91 
Mismatch, in castings, 35 

in die forgings, 36-38 
"Missile Maker Lathe," 265-266 
Mistake-proofing the fixture, 69 
Module, in gears, 102 
Molded plastic parts, draft on, 40 

shrinkage in, 40 

tolerances for, 40-41 
Mounting of press-fit bushings, 156- 

Movable milling fixtures, 2 53-2S4 
u (coefficients of friction) (Table), 1 06 
Multiple clamping, 140-146 
Multiple milling, 245 
Multiple-spindle drill heads, 164, 2 36 

Natural stress relief, 5 

N/C drilling machines, tooling for, 

N/C machine tools, dual fixturing for 

fixtures for, 1, 187, 257-25 8, 259, 

reference point for operations with, 
Nested, single, double, fully, 25 
Nesting, 25-26, 28, 29, 43-44, 93 

three-dimensional, 43-44 
N. I, J. b\ C. M. (National Institute of 
Jig and Fixture Component 
Manufacturers), 194 
Nodular cast iron, 23 
Non-standard bushings, 154, 163-164, 

Normalizing, of castings, 179 

ofweldments, 182 
Nuts, acorn, 116, 195, 203-204 
coupling, 195 
flange. 195,205 
knurled lock, 195 
materials for, 23 
speed, 117 
T-, 171, 195 
wing-, 1 16, 148 

Open jig, 170, 221-224, 227, 231-232 
Operating time for clamping devices 

(Table), 1 1 5 
Operating with drill jigs, 227-229 
Operations plan, manufacturing, 5 
Operations, sequence of, 5 
Operator criteria, manipulation and, 9 
Outboard bearing, 60 
Overdefining, 25, 53 
Overrunning clutch, 282 

Package clamping, 141-142 

Pads, 44, 206, 208, 222, 225-226, 227 

locating, 48-51, 208 

repair of worn, 45 

rest, 48-51,208 

swivel, 201 

tooling ball, 217 
Pallet fixtures, 58, 293-294 
Pallets for transfer machines, 293-294 
Pantograph system linkages, 97, 100- 

Parallels, materials for, 23 
Parameters, machining, 5 
Part, (def.) 25 

removal of, 76-80 
Patented fixtures and components, 195 
Permanent magnets, 81, 131 
Phenolics, 23-24, 44, 279 
Pilot pins, 210 
Pilots, 2 82 

Pin-handle screws, 116-117 
Pin locators, floating, 212-213 
Pins, captive, 214-215, 225 

"cross" and crossed diamond, 56-57 

detent, 214 

diamond, 56-57, 210, 254-256 

double action, 214 

dowel, 50-51, 173, 174 

L-, 213-215 

locating, 48,210-215,221,222 

materials for dowel, 22, 174 

materials for fulcrum, 23 

pilot, 210 

retractable, 255-256 

shot, 57,293 

single-action, 2 14 

T-, 213-215 

tapered, 50 
Pitch in gears, 102 
Placement of jig bushings, 229-231 
Planing fixtures, 272 
Planing, run-in and run-out distance 

in, 272 
Plastic and steel ball knobs, 201-202 



Plastic drill jigs, 24 

Plastic fillings for hydraulic equalizers, 

146, 290 
Plastic parts, draft on molded, 40 
shrinkage in, 40 
tolerances for, 40-41 
Plastics, 23-24, 164, 168, 172 

tolerances for prefabricated shapes, 
Plastic sheets, laminated, 24 
Plastic tooling, 23-24, 184, 211,213, 
laminates in, 23-24 
repair of, 24 
Plate, jig (i.e.: jig-plate), 219-221, 224, 

Plate jig (i.e.: plate-jig), 170, 219-221, 

Plate-type equalizer 138 
PlexiglasCH 83 
Plunger actuators, 2 1 6 
Plungers, 122, 137, 139, 140, 143, 
14S, 147-149, 209-213, 215-217, 
4S-degree, 137, 139 
indexing, 216-217 
spring, 2 1 7 
Plywood, application of, 24 
Pneumatic fixtures, 287-290 
Polyesters, 23-24 
Polyuretliane foam, 24 
Polyvinyls, 23-24 
Portable power tools, bushings for, 

Positioner, boring, 208 

welding, 276, 280 
Pot type lathe fixture, 172, 262 
Potted bushings, 168, 184 
Potting compounds, 24 
Power cylinder, 287 
Power tools, bushings for portable, 

Prefabricated shapes, tolerances for 

plastic, 41 
Preparation for locating, 32-41 
Preplanning of fixture design, 5-8 
Prcposttioner, receiver as a, 81 
Press- fit bushings, 155, 156, 158 

mounting of, 156-159 
Press products, tolerances for, 39 
Pressure clamps, dual-, 289, 290 
Pressure intensifier, 290 
Preventing rotation, stop block for, 

Production boring machines, 266, 269 
Production quantities, classification 

of, 219 
Profile milling fixtures, 252-2S3 
Profit estimate, 295-296 
Proprietary fixtures and components, 

Pull broaching, fixtures for, 274-275 
Pump jig, 170, 240,281 
Punched parts, foolproofing of, 76 
Push-pull toggle clamps, 216-217 

Quantities, classification of produc- 
tion, 2 1 9 
Quarter-turn, knob, 125 

screw, 123-124,201 
Quick-acting, clamps, 127-128, 239, 

devices, 4, 206 

screw components, 202 
Quick locking, knobs, 202 

levers, 202 
Quick operation, designs for, 1 1 5 

Radial drill, 227-228, 268 

Radial fixtures, 254 

Radial locating error, 91 

Radial locating, keys and keyseats for, 

Radial locators, 29, 54 

Radians, measuring angles In (A pp.), 

Raiser blocks, materials for, 23 
Rapid indexing, 60 
Rate setting, 283 
Receiver as a pre positioner, 81 
Receivers, 80-81 
Recentering device, lathe fixture 

with, 64-67 
Redundancy, 137, 147 
Redundant supports, 61 
Reference point for operations with 

N/C machine tools, 242 
Relief for dirt and burrs, 84-86 
Removable clamp assemblies, 205 
Removal of chips, 83-84, 247. 284 
Removal of part, 76-80 
Renewable bushings, installation of, 

locking clamps for, 161-162 
Renewable (loose) bushings, 154, 155 
Renewable wearing bushings, ) 54-155, 

Repair of plastic tooling, 24 
Repair of worn pads, 45 
Residual stresses, 33, 76, 178, 179 
Resins, laminating, 24 
Rest buttons, 48, 208 
Rest pads, 48-51, 208 
Retainer bushings, 208 
Retainers, 208, 211 
Retractable pins, 255-256 
Reversible jig, 2 3 1 
Rockers, 137-138 

for jigs, 235 
Roller bearings, 59 

Rollers for multiple clamping, 144-14S 
Rotating fixture, 254-255, 256-257 
Rotational locators, 29 
Rovings, glass fiber, 24 
Rules for dimensioning of cast fix- 
tures, 177-179 
Run-in and run-out distances, in milling, 

in planing, shaping, and slotting, 272 

SAE steels, 20-21 

Safety feature for air-operated clamps, 

242, 244, 288-289 
Sapphire locators, synthetic, 45 
Scissor-type linkages, 97 
Screw bushings, 95-97 
Screw clamp assemblies, 202-205 
Screw components, quick-acting, 202 
Screws, clamping, 107, 108, 115-117, 
147, 202-205 

floating, 139 

forces from (Table), 1 08 

half-turn, 201 

hand knob, 147, 199,251 

jack, 117-118,201,224 

knurled head, 200 

materials for, 22, 1 16 

pin-handle, 116-117 

quarter-turn, 123-124,201 

socket head, 1 1"6 

swing jack, 117-118 

swivel, knob, 199-200, 205 

thumb-, 119, 123, 147 

torque head, 117, 147, 201 

wing-, 147 
Screw thread, double or triple,-135 

for ejectors, 79 

square, 125 
Seals for dirt and dust, 86, 210 
Sections, cast iron fixture stock, 217 
Selection of steel materials, 21-23, 167 
Self-centering chucks, 87, 89, 90, 102 
Self-locking cams, 106, 109-111 
Self-locking ranges for eccentric cams 

(Table), 111 
Self-locking wedges, 106 
Sequence of operations, 5 
Serrated clamp straps, 207-208 

Set-up gages, 1 51 

Setting block, tool, 2, 1 51, 244, 292 

Setting gages, 151, 253 

Shapers, field (for magnets), 1 32 

Shapes, structural, 175, 181-182, 183, 

Shaping fixture, 272 
Sheets, laminated plastic, 24 
Shields for dirt and dust, 86, 210 
Shoe, swiveling, 137, 147, 199 
Shot pins, 57, 293 
Shoulder screws, 1 97 
Shoulder tooling ball, 2 1 7 
Shrinkage and warpage of castings, 33 
Shrinkage in molded plastic parts, 40 
Shrinkage tolerances for die forgings, 

Shrinkage, welding, 276 
Side-cutting-edge angle (SCEA), 303 
Side stops, 48 
Sighting, locating by, 42-43 
Sine bars, 285 
Sine fixture keys, 195 
Single, (def.) 25 
Single-action pins, 214 
Single centering, 87 
Single, double and fully nested, 25 
Single locking levers, 202 
Sintered (cemented) carbides, 23, 167, 

Size adjustment, lathe fixtures with, 

Sliding conical locator, 94-95 
Sliding point locator, split-cylinder 

type, 63-64 
Sliding V-block, 92-93, 101 
Sliding wedges, 97-98 
Slip bushings, 15 5, 159-163 
bayonet-type locking device for, 

integral locking tabs for, 162-163 
locking devices for, 1 60 
Slots, T-, 171, 195, 285-287, 292 
Slotted hole locators, 213 

Slotted shoulder screws, 197 
Slotting fixtures, 272-273 
Socket-head screws, 1 16 

Socket shoulder screws, 197 

Space in fixtures, 68, 76, 172, 227 

Speed-ball handles, 201 

Speed handle, 205 

Speed nut, 11-7 

Spheres for multiple clamping, 144-145 

Spherical buttons, 208 

Spherical flange nuts, 195 

Spherical locators, 52, 90, 102, 208 

Spherical washers, 137, 196, 203, 206 

Spiral, Archimedes, 1 10 

Spiral cams, 1 10 

Split-cylinder type of sliding point 
locator, 63-64 

Split V-block, 97 

Spring jack locks, 209-210 

Springback, 39 

Spring loaded jack, 246 

Spring plungers, 217 

Springs, 137, 138, 139, 140, 143, 147, 
203, 205, 206, 209-210, 214, 240, 
materials for, 22 

Spring stops, 2 1 7 

Spur gear, 103 

Square screw thread, 125 

Stability, 60, 245 

Stabilizing treatment of castings, 179 

Stainless steels, 2 1 

Standard and commercial fixture com- 
ponents, 4, 194-218 

Standardization of fixture components, 

Standard steels, 20-21 

Stand or base, for a jig, 234 

Star hand knobs, 201 

Statically determinate system, 61, 147 



Statically indeterminate system, 61 
Stationary press-fit bushings, 154 
Steel, 20-2 3, 167 

alloy, 21, 67 

ball knobs, 20 1-202 

Castings, 23 

inserts for lathe chucks, 208 

"liquid," 24 

material selection, 21-23, 167 

Stainless, 2 1 

standard, 20-2 I 

tool, 21, 167, 183 
Step blocks, 207-208 
Stock sections, east iron fixture, 217 
Stop block for preventing rotation, 228 
Stop buttons, 41 
Stop collar on a .frill, 159 
Stops, 42 

side, 48 

spring, 217 
Straight line indexing, S3 
Straps, 105, 1 13-1 IS, 118-120, 138, 
202-205, 207-208, 222, 223, 226, 

angle, 113, 120 

clamp, 207-208 

materials for clamping, 23 
Stress and deformation analysis in fix- 
tures, (App.), 302-303 
Stresses, machining, 179 

residual, 33, 76, 178, 179 
Stress relief, 5, 179 
String milling, 141, 244-245, 252-253 
Strips, 44 
Structural design of fixtures (App.), 

Structural integrity, 2 
Structural shapes, 175, 181-182, 183, 

218, 279 
Stub boring bars, 265, 268, 269 
Studs, 195,203-204 
Subassembly fixtures, welding, 278 
Sub-bases, 281, 284-286 
Supports, 19 

adjustable, 209-210 

compatible, 61 

incompatible, 61 

intermediate, 147-150,209-210 

redundant, 61 
Surface plate, magnetic, 130-132 
Swing bolts, 195 
Swing clamp assemblies, 205 
Swing C -washers, 197 
Swinging leaf, 145, 123-125, 205, 226 
Swing jack screws, 117-118 
Swivel pads, 201 
Swiveling shoe, 137, 147, 199 
Swivel screws, knob, 1 99-200, 205 
Symmetry considerations, 69-71 
Synthetic sapphire locators, 45 

Tacking fixture for welding, 278 
Tapered holes, tooling for, 267-268 
Tapered pins, 50 
Target marks, 58 
T-bolts, 171. 195 
Template bushings, 168-169 
Template jig, 227 

Templates, 1 SI, 153, 186, 19S, 2 S3, 

254, 255,272 
Template tooling, 168 
Thin-wall bushings, 163 
Thread, double or triple, 135 
Threaded adjustable locating buttons, 

Threaded arbors and mandrels, 264-265 
Threaded buttons, 47, 201, 208 
Threaded drill bushings, 95-97, 163-164, 

229, 233 
Three-dimensional nesting in a castable 

material, 43-44 
3-2-1 Locating principle, 26-27, 245, 

Thumb-screws, 119, 123, 147 
Titanium carbide for bushings, ferrous, 

T-nuts, 171, 195 
Toggle clamps, 111-113, 12S, 128-129, 

136, 216-217, 243, 279 
Tolerances, 32-41, 189-1 91, 242, 247, 
251, 280,299-301 

for castings, 33-35 

for die forgings (Table), 36-38 

for forgings, 35-38 

for heat treated parts, 40 

for machined parts, 39-40 

for mill products, 39 

for molded plastic parts, 40-41 

for plastic prefabricated shapes, 41 

for press products, 39 

for torch-cut parts, 38-39 

for weldments, 38 

incompatible, 27 

toolroom, 191 
Tooling ball, 217 
Tooling ball pad, 217 
Tooling, castable, 211 -213 

template, 1 68 
Tooling for N/C drilling machines, 240- 

Tooling for tapered holes, 267-268 
Tooling holes, 57, 170, 252, 293 
Tooling plate, aluminum and magnesium 

23, 168, 183 
Too [maker's construction ball, 217 
Toolroom tolerances, 191 
Tool setting block, 2, 151, 244, 292 
Tool steel, 21, 167, 183 
Torch-cut parts, tolerances for, 38 
Torque head screws, 117, 147, 201 
T-pins, 213-215 
Tracer milling, 2 5S 
Transfer fixtures, 58, 292-294 
Transfer machines, 57-58, 266, 292- 

pallets for, 293-294 
Transfer of dimensions, 1 89, (App.) 

Treatment of castings, stabilizing, 1 79 
Triangular relief, locator with, 52-53 
Triple screw thread, double or, 1 3S 
Trunnions, 60, 235, 253 
T-slots, 171, 195, 285-287, 292 
Tubing, in welded fixtures, 182 
Tumbling jig, 227, 22 8, 283, 284 
Turnbuckle principle, 97 

Turning fixture, converting a chuck into 

a, 208 
Twist drills, 82, 154, 240, 303 
chisel edge on, 154 

Unhardened locators, 46 
Uniformity of Castings, 35 
Universal drill jigs, 170, 281-284 
Universal fixtures, 284-287 

(def.) 2, (def.) 281 
Universal jigs, 208 

locking mechanism for, 282 
Unloading and loading the part, 68-81 
Unmachined surfaces, locating from, 

USA Standard bushings, 1 54- 1 56 

Vacuum clamping, 133, 269 
V-blocks, 26, 28, 90-93, 103, 130, 132, 
136, 142, 204, 236, 248, 251, 262, 
sliding, 92-93, 101 
split, 97 
Vibration damping, 23 
Vise jaws, detachable, 130, 244 

materials for, 22 
Vises, adapters for, 129-130, 2 35-236, 
air-operated machine, 170 
machine tool, 90, 244, 252, 281 
Volume of chips, 82 

Warpage of castings, shrinkage and, 33 
Washers, hardened, 23, 

spherical, 137, 196, 203, 206 
Wear on indexing components, 58-60 
Wear on locators, 4S-46 

adjustment for, 63-67 
Wedges, clamping, 64-67, 105-106, 122, 

self-locking, 106 

sliding, 97-98 
Welded fixture bodies, 45, 172, 173, 

Welded fixtures, tubing in, 182 
Welded joints, 179-180 
Welding, design rules for, 181>182 

fixtures, 275-280 

lathe, 280 

magnetic flux in, 278 

positioner, 276, 280 

shrinkage in, 276 

subassembly fixture for, 278 

tacking fixture for, 278 
Weldments, distortion in, 38, 275-278 

normalizing of, 182 

tolerances for, 38 
Wide bushings, I 65 
Wing-nuts, 116, 148 
Wing-screws, 147 
Without bushings, drilling, 242 
Worn pads, repair of, 45 

Yoke, 138, 235 

Zero, floating, 292 
Zinc base alloys, 44